p2.1-2 physics: forces
TRANSCRIPT
AQA GCSE Additional Science
Physics 2 Forces
2.1 Forces and their effects 2.2 The kinetic energy of objects speeding up or slowing down
Steve Bishop
August 2013
Updated May 2014
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Table of Contents P2.1 Forces and their effects ................................................................................................ 3
Forces: true or false? ......................................................................................................... 4
P2.1.1 Resultant forces ..................................................................................................... 5
P2.1.2 Forces and motion.................................................................................................. 6
Motion and Graphs ............................................................................................................ 7
P2.1.3 Forces and braking ............................................................................................... 10
P2.1.4 Forces and terminal velocity ................................................................................. 11
P2.1.5 Forces and elasticity ............................................................................................. 13
P2.2 The kinetic energy of objects speeding up or slowing down ........................................ 16
P2.2.1 Forces and energy ............................................................................................... 16
Measuring human power ................................................................................................. 17
Power .............................................................................................................................. 18
Work ................................................................................................................................ 18
P2.2.2 Momentum ........................................................................................................... 21
GCSE-style questions: forces ............................................................................................. 25
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P2.1 Forces and their effects Forces can cause changes to the shape or motion of an object. Objects can move in a
straight line at a constant speed. They can also change their speed and / or direction
(accelerate or decelerate). Graphs can help us to describe the movement of an object.
These may be distance–time graphs or velocity–time graphs.
You should be able to:
■ interpret data from tables and graphs relating to speed, velocity and acceleration
■ evaluate the effects of alcohol and drugs on stopping distances
■ evaluate how the shape and power of a vehicle can be altered to increase the vehicle’s top
speed
■ draw and interpret velocity–time graphs for objects that reach terminal velocity, including a
consideration of the forces acting on the object
Key words
Force
Motion
Resultant
Stationary
Velocity
Acceleration
Mass
Gradient
Speed
Stopping distance
Reaction time
Friction
Kinetic energy
Terminal velocity
Elastic
Potential energy
Work
Power
Mass
Momentum
Conservation of momentum
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Forces: true or false?
True False Not sure
If something is moving there is a force acting on it
To move something we need a force
A force is needed to change the shape of something
A force is a type of energy
Weight is a force
Mass is a force
Stretching a spring needs a force
A frictionless car does not need a force to keep it moving
Gravity is a type of force
Friction is a type of force
Forces always occur in pairs
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P2.1.1 Resultant forces
a) Whenever two objects interact, the forces they exert on each other are equal and opposite.
b) A number of forces acting at a point may be replaced by a single force that has the same
effect on the motion as the original forces all acting together. This single force is called the
resultant force.
c) A resultant force acting on an object may cause a change in its state of rest or motion.
d) If the resultant force acting on a stationary object is:
■ zero, the object will remain stationary
■ not zero, the object will accelerate in the direction of the resultant force.
e) If the resultant force acting on a moving object is:
■ zero, the object will continue to move at the same speed and in the same direction
■ not zero, the object will accelerate in the direction of the resultant force.
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P2.1.2 Forces and motion
a) The acceleration of an object is determined by the resultant force acting on the object and the mass of the object. F
or F = m × a m a
b) The gradient of a distance–time graph represents speed.
c) Calculation of the speed of an object from the gradient of a distance–time graph. d) The velocity of an object is its speed in a given direction. e) The acceleration of an object is given by the equation:
f) The gradient of a velocity–time graph represents acceleration.
g) Calculation of the acceleration of an object from the gradient of a velocity–time graph. h) Calculation of the distance travelled by an object from a velocity–time graph.
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Motion and Graphs
Velocity-time graphs
The slope (gradient) of a velocity-time graph tells us the acceleration of a moving object.
The steeper the slope of the graph the greater its acceleration.
The area under the graph represents the distance travelled
Time (s)0
Velocity (m/s)
accelerating (speeding up)
steady speed
decelerating (slowing down))
stationary
Area represents
distance travelled
Questions
1. Here is the velocity -time graph for a bike. In which section of the journey was it (a)
stationary (b) accelerating (c) decelerating (d) moving fastest?
0 Time (s)
Velocity (m/s)
A
BC
D
E
F
G
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Example
The graph shows how the speed of a car changed as it travelled along a straight road. What
was the acceleration of the car?
0 Time (s)
Velocity (m/s)
30
10
2 10
Step 1: Choose two (well separated) points on the graph
Step2 Work out the change in speed between the two points
change in speed = final speed - initial speed
= 30 -10 = 20 m/s
Step 3 Work out the time interval between the two points
time taken = final time - initial time
= 10 - 2 = 8 s
Step 4 Work out the acceleration of the car
Acceleration = change in speed / time taken = 20 / 8 = 2.5 m/s2 answer
Questions
1. The graph shows the speed of a train along a section of its journey. (a) What is its initial
speed? (b) What is its speed after 50 s? (c) What is its acceleration?
0 Time (s)
Velocity (m/s)
20
10
50
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2. Sketch a velocity-time graph to show the following journey:
A train sets off from a station and accelerates to a steady speed. It maintains this
speed for a while, until it passes an amber signal. The driver then brakes so that it
slows down gradually; the next signal is green, so the driver speeds up again.
SUMMARY OF MOTION GRAPHS
Fill in the missing words:
The _____________ of a distance-time graph tells us how _____________ something is
moving.
The steeper the slope of a distance-time graph, the _____________ the speed it
represents
The _____________ of a velocity-time graph tells us the _____________ of a moving
object.
The steeper the slope of a velocity-time graph, the _____________ the acceleration it
represents.
The area under a velocity-time graph represents the _____________.
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P2.1.3 Forces and braking
a) When a vehicle travels at a steady speed the resistive forces balance the driving force. b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance. c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).
Stopping distance = Thinking distance + Braking distance
d) A driver’s reaction time can be affected by:
tiredness drugs and alcohol.
e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases. f) A vehicle’s braking distance can be affected by:
adverse road and weather conditions and poor condition of the vehicle (eg worn tyres, worn brakes)
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P2.1.4 Forces and terminal velocity
a) The faster an object moves through a fluid the greater the frictional force that acts on it.
b) An object falling through a fluid will initially accelerate due to the force of gravity.
Eventually the resultant force will be zero and the object will move at its terminal velocity
(steady speed).
c) Draw and interpret velocity-time graphs for objects that reach terminal velocity, including a
consideration of the forces acting on the object.
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The above graph shows the velocity of a ball bearing being dropped in a long tube of oil. Label the above graph to show the terminal velocity and the initial acceleration. Describe what is happening between A and B Describe what is happening between B and C Describe what is happening between C and D From http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/forces/forcesvelocityrev2.shtml
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Stage Diagram Forces Motion
1
His weight is much greater than the air ________
The skydiver is ________
2
As the skydiver gains in speed, the air resistance increases until it becomes equal to his ______
The skydiver reaches a constant speed, this is known as his t_____velocity
3
The skydiver opens his parachute and so the air resistance becomes much greater than the weight due to the large surface area
The skydiver’s speed _________ dramatically
4
As the skydiver slows down, the ____ _________ decreases until it becomes equal to the weight
The skydiver reaches a _______speed at a rate which is safe enough to land
1
2
3
4
The graph shows the
motion of a skydiver
from when he leaves
the aeroplane until
just before he lands.
velocity
time
Terminal Velocity
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Mass and weight d) Calculate the weight of an object using the force exerted on it by a gravitational force:
W = m × g Example A person has a mass of 80kg. What is his weight? Using the formula and knowing that on earth g = 9.8 m/s2: Weight = 80 x 9.8 = 784 N Now try this 1. A person has a mass of 70 kg. What is his weight? 2. An object has a mass of 980 N. What is its weight? 3. On the moon g = 1.6 N/kg. Work out the masses of each of these objects on the moon: (a) 10 kg (b) 84 kg (c) 230 kg (d) 65 kg
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P2.1.5 Forces and elasticity
a) A force acting on an object may cause a change in shape of the object. b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy. c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.
d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded:
F = k × e
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P2.2 The kinetic energy of objects speeding up or slowing down
When an object speeds up or slows down, its kinetic energy increases or decreases. The forces which cause the change in speed do so by doing work. The momentum of an object is the product of the object’s mass and velocity. You should be able to: ■ evaluate the benefits of different types of braking system, such as regenerative braking.
■ evaluate the benefits of air bags, crumple zones, seat belts and side impact bars in cars.
Key words Force Energy Work done Power
Potential energy Kinetic energy Momentum
P2.2.1 Forces and energy
a) When a force causes an object to move through a distance work is done. b) Work done, force and distance, are related by the equation:
W = F × d
c) Energy is transferred when work is done. d) Work done against frictional forces. e) Power is the work done or energy transferred in a given time.
Power = work done time = t
Fd
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Measuring human power
One way to estimate your own power is to time yourself running up a flight of stairs.
You will need to know your mass in kg and the vertical height of the stairs.
To calculate the power, we need to work out the energy gained per second.
If the mass of a person is 70 kg and the height of the stairs is 10 m, then gravitational
potential energy gained
= mgh
= 70 kg 10 10 m
= 7000 Nm (= 7000 J)
This is the amount of energy transferred from chemical energy in the person’ muscles into
gravitational energy.
Power = energy transferred/ time taken
= 7000 J / 15 s
= 466.67 J/s (or watts)
The person’s power output during this task is 467 W.
Name Force
(N)
Height
(m)
Average
time
Energy (J) Power
(W)
Power range
(W)
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Power
Two motors can lift a load of 1 kg from the floor to the bench. One motor can lift it quicker
than the other.
Which has done the most work?
Which motor has the most power?
The rate at which something can do work is called power
Power = (work done) ÷ time taken
When work is done energy is transferred from one form to another. So we can write
Power = (energy transformed) ÷ (time taken)
Power is measured in Watts (W). 1 W is equivalent to 1 joule per second (J/s)
Work
Work is done only when a force moves an object
WORK
Work done = force x distance F d
The joule (J) is the unit of energy. It is equal to a force of 1 newton pulling (or pushing)
through a distance of 1 m.
Question
A woman lifts a parcel weighing 7 N onto a shelf 2 m high. How much work has she done?
Answer
1. Write down equation: Work done = force x distance
2. Write down what you know:
force = 7N
distance = 2m
3. Rearrange equation if necessary
4. Plug in numbers and do calculation: 7 N x 2 m = 14 Nm
5. Check units: 14 Nm = 14 J
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Questions
1. A man weighs 800N climbs 6 stairs vertically, walking up to his flat. If each step is 15 cm
high how much work does he do?
2. 20 J of work are done in throwing a ball upwards.
a) How much kinetic energy is given to the ball?
b) What is the potential energy of the ball at its highest point?
c) If the highest point is 5 m what is the mass of the ball?
3. If an object gained 150 J of kinetic energy in travelling 0.5 m:
a) how much work was done on the object?
b) What was the average force acting on it?
c) If a force of 400 N had acted over the same distance what would the object have gained in
kinetic energy?
4. How much work is done when a concrete block weighing 20 000 N is lifted 25 m upwards
by a crane?
5. A mountaineer weighs 600 N. He carries a pack weighing 150 N. How much work does
he do in climbing a mountain 3000 m high?
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f) Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field
Ep = m × g × h
Example A 80 kg man is lifted by a crane 2 m. Take g = 10 N/kg, found out the gain in potential energy Using Ep = m × g × h m = 80 kg, g = 10 N/kg and h = 2 m Then Ep = 80 × 10 × 2 = 1600 Nm = 1600 J g) The kinetic energy of an object depends on its mass and its speed.
Ek = ½ m × v2 Example Find the kinetic energy of a 60 kg man running at 10 m/s. Using Ek = ½ m × v2 m = 60 kg, v = 10 m/s Then Ek = ½ × 80 × 102 = 40 × 100 = 4000 J Now try these 1. Calculate the kinetic energy of each of the following: (a) a stone of mass 0.1 kg moving at 20 m/s (b) a brick of mass 5 kg falling at 10 m/s (c) a bee of mass 1 g flying at 5 m/s [watch the units!] (d) a car of mass 100kg travelling at 30 m/s [watch the units] 2. Calculate the change in gravitational potential energy in each of the following: (a) A book of mass 1 kg is lifted up 0.5 m form the floor to a table. (b) A rocket of mass 500 kg rises 500m into the air (c) A stone of mass 0.2 kg falls from the top of a 60 m high cliff.
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P2.2.2 Momentum
a) Momentum is a property of moving objects.
p m × v b) In a closed system the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.
Example
(a) A car has a mass of 1100 kg and travels at 30 m/s. Find the momentum of the car.
Using p = mv
= 1100 x 30 = 33 000 kg m/s
(b) The same car hits another car of mass 1000 kg which is stationary. Find the total momentum
before and after the collision.
If the car is stationary then it will have a velocity = 0. If v= 0 then the momentum will also be zero.
The momentum before the collision will be: 33 000 + 0 = 33 000 kg m/s
The momentum after the collision will also be 33 000 kg m/s
Now try these
Find the momentum of a 1100 kg car travelling at:
(a) 50 m/s (b) 33 m/s (c) 50 km/h (don’t forget to convert the velocity to m/s)
Example
Two trucks are moving in the same direction along the track of an adventure park ride. One
has a velocity of 8 m/s and a mass of 500 kg and the other with twice the mass has a
velocity of 6 m/s. They collide and link together. What is their new velocity?
Solution
The initial momentum of the first truck = m1 v1 = 500 8 = 4000
The initial momentum of the second truck = m2 v2 = 1000 6 = 6000
The total momentum before the collision = 4000 + 6000 = 10 000
The total momentum after the collision must also be 10 000
After the collision the two trucks stick together, so the combined mass will be
m1 + m2 = 500 + 1000 = 1500
The momentum will then be
(m1 + m2) vnew = 10 000
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1500 vnew = 10 000
Giving
vnew = 1500
10000= 6.67 m/s
Now try these
1. A vehicle travelling at 40 m/s has a mass of 1250 kg. Calculate its momentum.
2. A car with a momentum of 60000 kg m/s has a mass of 2500 kg. Find its velocity.
3. Two cars are moving in the same direction. One has a velocity of 5 m/s and a mass
of 1000 kg and the other with a mass of 15000 kg has a velocity of 10 m/s. They
collide and link together.
(a) what is the momentum of the two cars before the collision?
(b) What is the momentum of the two cars after the collision?
(c) What is their new velocity?
4. A car of mass 1250 kg is waiting at a traffic light with its hand brake off. A car of
mass 1500 kg travelling at 25 m/s collides with the stationary car. The two cars link
together and move forward.
(a)What is their velocity after the accident?
(b) What is the total kinetic energy before the collision?
(c) Is this an elastic or inelastic collision?
(d) What will be the kinetic energy after the collision?
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Car safety
Crumple Zone
The car is designed so that the structure of the car will give way during a collision. The metal
of the car will dent, bend and fold during a collision which increases the amount of time it
takes the car to stop.
The parts of the car that do this (the front and the back) are called crumple zones.
Seatbelts
Car seatbelts protect people in two ways during a crash. The seatbelt prevents the person
being thrown about in the car, possibly through the windscreen or hitting themselves on the
steering wheel or other objects.
The seatbelt also stretches a little, while restraining the person during a crash. The
stretching increases the amount of time it takes the person to stop.
Airbags
Airbags are bags which inflate very quickly during a crash. They provide a softer surface
(like a pillow) to prevent the people hitting themselves on hard objects.
They are designed to be used with a seatbelt. An airbag will give way a little when a person
hits it and this gives an extra increase to the amount of time it takes the person to stop.
Bubble wrap packaging has the same effect and is used to protect objects that are being
transported.
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Extended question
In 1983 it became a legal requirement to wear a seatbelt whilst travelling in a car in
the UK. Since then new car designs have developed to increase the safety of the
driver and passengers. Outline these safety designs. You should include:
• A description of the safety features of a vehicle • An explanation of how they protect the people in the car • The Physics principles in action when the vehicle stops suddenly or crashes • Any ideas you have on future vehicle safety features • Diagrams to support your description. ………………………………………………………………………………………………..
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Use another sheet of paper if you need it.
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GCSE-style questions: forces
Q1. The diagram shows the horizontal forces acting on a car of mass 1200 kg.
(a) Calculate the acceleration of the car at the instant shown in the diagram.
Write down the equation you use, and then show clearly how you work out your answer and give the unit.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
Acceleration = ............................. (4)
(b) Explain why the car reaches a top speed even though the thrust force remains constant at 3500 N.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................ (3)
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(c) The diagram shows a car and a van.
The two vehicles have the same mass and identical engines.
Explain why the top speed of the car is higher than the top speed of the van.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................ (4)
(Total 11 marks)
Q2.
Five forces, A, B, C, D and E act on the van.
(a) Complete the following sentences by choosing the correct forces from A to E.
Force ................ is the forward force from the engine.
Force ................ is the force resisting the van’s motion. (1)
(b) The size of forces A and E can change. Complete the table to show how big force A is compared to force E for each motion of the van. Do this by placing a tick in the correct box. The first one has been done for you.
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MOTION OF VAN FORCE A SMALLER THAN FORCE E
FORCE A EQUAL TO FORCE E
FORCE A BIGGER THAN FORCE E
Not moving
Speeding up
Constant speed
Slowing down
(3)
(c) When is force E zero?
.................................................................................................................................... (1)
(d) The van has a fault and leaks one drop of oil every second. The diagram below shows the oil drops left on the road as the van moves from W to Z.
Describe the motion of the van as it moves from:
W to X ........................................................................................................................
X to Y .........................................................................................................................
Y to Z .......................................................................................................................... (3)
(e) The driver and passengers wear seatbelts. Seatbelts reduce the risk of injury if the van stops suddenly.
backwards downwards force forwards mass weight
Complete the following sentences, using words from the list above, to explain why the risk of injury is reduced if the van stops suddenly.
A large ........................................ is needed to stop the van suddenly.
The driver and passengers would continue to move ............................................... .
The seatbelts supply a ........................................ force to keep the driver and passengers
in their seats. (3)
(Total 11 marks)
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Q3. The diagram shows a high jumper.
In order to jump over the bar, the high jumper must raise his mass by 1.25 m. The high jumper has a mass of 65 kg. The gravitational field strength is 10 N/kg.
(a) The high jumper just clears the bar.
Use the following equations to calculate the gain in his gravitational potential energy.
weight = mass × gravitational field strength
(newton, N) (kilogram, kg) (newton/kilogram, N/kg)
change in gravitational potential energy = weight × change in vertical height
(joule, J) (Newton, N) (metre, m)
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Gain in gravitational potential energy .................... J (4)
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(b) Use the following equation to calculate the minimum speed the high jumper must reach for take-off in order to jump over the bar.
kinetic energy = × mass × [speed]2
(joule, J) (kilogram, kg) [(metre/second)2, (m/s)2
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
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Speed .................... m/s (3)
Q4. The Highway Code gives tables of the shortest stopping distances for cars travelling at various speeds. An extract from the Highway Code is given below.
thinking distance + braking distance = total stopping distance
(a) A driver’s reaction time is 0.7 s.
(i) Write down two factors which could increase a driver’s reaction time.
1 .........................................................................................................................
2 ......................................................................................................................... (2)
(ii) What effect does an increase in reaction time have on:
A thinking distance; ..........................................................................................
B braking distance; ...........................................................................................
C total stopping distance? .................................................................................
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(3)
(b) Explain why the braking distance would change on a wet road.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
.................................................................................................................................... (2)
(c) A car was travelling at 30 m/s. The driver braked. The graph below is a velocity-time graph showing the velocity of the car during braking.
Calculate:
(i) the rate at which the velocity decreases (deceleration);
...........................................................................................................................
...........................................................................................................................
Rate .......................... m/s² (2)
(ii) the braking force, if the mass of the car is 900 kg;
...........................................................................................................................
...........................................................................................................................
Braking force ............................... N (2)
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(iii) the braking distance.
...........................................................................................................................
...........................................................................................................................
Braking distance .............................. m (2)
(Total 13 marks)
Q5. A sky-diver jumps from a plane.
The sky-diver is shown in the diagram below.
(a) Arrows X and Y show two forces acting on the sky-diver as he falls.
(i) Name the forces X and Y.
X ................................................. Y .......................................................... (2)
(ii) Explain why force X acts in an upward direction.
........................................................................................................................... (1)
(iii) At first forces X and Y are unbalanced.
Which of the forces will be bigger? ....................................... (1)
(iv) How does this unbalanced force affect the sky-diver?
...........................................................................................................................
........................................................................................................................... (2)
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(b) After some time the sky-diver pulls the rip cord and the parachute opens.
The sky-diver and parachute are shown in the diagram.
After a while forces X and Y are balanced.
Underline the correct answer in each line below.
Force X has
increased / stayed the same / decreased.
Force Y has
increased / stayed the same / decreased.
The speed of the sky-diver will
increase / stay the same / decrease
(c) The graph below shows how the height of the sky-diver changes with time.
(i) Which part of the graph, AB, BC or CD shows the sky-diver falling at a constant speed?
................................................ (1)
(ii) What distance does the sky-diver fall at a constant speed?
Distance .............................. m (1)
(iii) How long does he fall at this speed?
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Time .................................... s (1)
(iv) Calculate this speed.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
Speed .............................. m/s (2)
(Total 14 marks)
Q6. A car travels along a level road at 20 metres per second.
(a) Calculate the distance travelled by the car in 4 seconds.
(Show your working.)
.....................................................................................................................................
.....................................................................................................................................
..................................................................................................................................... (3)
(b) When the brake pedal of the car is pushed, brake pads press against very hard steel discs.
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The force of friction between the brake pads and the steel discs gradually stops the car.
What two effects does using the brakes have on the brake pads and wheel discs?
1 ..........................................................................................................................
2 .......................................................................................................................... (3)
(Total 6 marks)
Q7. A driver is driving along a road at 30 m/s. The driver suddenly sees a large truck parked across the road and reacts to the situation by applying the brakes so that a constant braking force stops the car. The reaction time of the driver is 0.67 seconds, it then takes another 5 seconds for the brakes to bring the car to rest.
(a) Using the data above, draw a speed-time graph to show the speed of the car from the instant the truck was seen by the driver until the car stopped.
(5)
(b) Calculate the acceleration of the car whilst the brakes are applied.
....................................................................................................................................
....................................................................................................................................
............................................................................. Answer = .................................... m/s2 (3)
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(c) The mass of the car is 1500 kg. Calculate the braking force applied to the car.
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Answer = .................................... N (3)
(d) The diagrams below show what would happen to a driver in a car crash.
(i) Explain why the driver tends to be thrown towards the windscreen.
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(ii) During the collision the front end of the car becomes crumpled and buckled. Use this information to explain why such a collision is described as “inelastic”.
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(iii) The car was travelling at 30 m/s immediately before the crash. Calculate the energy which has to be dissipated as the front of the car crumples.
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..........................................................................................................................
.......................................................................................................................... (8)
(Total 19 marks)
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Q8. A car driver sees a dog on the road ahead and has to make an emergency stop.
The graph shows how the speed of the car changes with time after the driver first sees the dog.
(a) Which part of the graph represents the “reaction time” or “thinking time” of the driver?
..................................................................................................................................... (1)
(b) (i) What is the thinking time of the driver? Time ........................ seconds (1)
(ii) Calculate the distance travelled by the car in this thinking time.
...........................................................................................................................
...........................................................................................................................
Distance ..................................... m (3)
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(c) Calculate the acceleration of the car after the brakes are applied.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Acceleration ............................................ (4)
(d) Calculate the distance travelled by the car during braking.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Distance ................................................ m (3)
(e) The mass of the car is 800 kg. Calculate the braking force.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Braking force ........................................ N (3)
(Total 15 marks)
Q9. The manufacturer of a family car gave the following information.
Mass of car 950 kg
The car will accelerate from 0 to 33 m/s in 11 seconds.
(a) Calculate the acceleration of the car during the 11 seconds.
.....................................................................................................................................
.....................................................................................................................................
..................................................................................................................................... (2)
(b) Calculate the force needed to produce this acceleration.
.....................................................................................................................................
..................................................................................................................................... (2)
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(c) The manufacturer of the car claims a top speed of 110 miles per hour. Explain why there must be a top speed for any car.
.....................................................................................................................................
..................................................................................................................................... (3)
(Total 7 marks)
Q10. When a car driver has to react and apply the brakes quickly, the car travels some distance before stopping. Part of this distance is called the “thinking distance”. This is how far the car travels while the driver reacts to a dangerous situation.
The table below shows the thinking distance (m) for various speeds (km/h).
Thinking distance (m) 0 9 12 15
Speed (km/h) 0 48 64 80
(a) On the graph paper, draw a graph of the thinking distance against speed. (2)
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(b) Describe how thinking distance changes with speed.
.....................................................................................................................................
..................................................................................................................................... (1)
(c) The time the driver spends thinking before applying the brakes is called the “thinking time”.
A driver drank two pints of lager. Sometime later the thinking time of the driver was measured as 1.0 seconds.
(i) Calculate the thinking distance for this driver when driving at 9 m/s.
...........................................................................................................................
...........................................................................................................................
Answer ............................................ m (1)
(ii) A speed of 9 m/s is the same as 32 km/h. Use your graph to find the thinking distance at 32 km/h for a driver who has not had a drink.
...........................................................................................................................
Answer ............................................ m (1)
(iii) What has been the effect of the drink on the thinking distance of the driver?
...........................................................................................................................
........................................................................................................................... (1)
(Total 6 marks)
Q11. A cyclist goes on a long ride. The graph shows how the distance travelled changes with time during the ride.
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(i) Between which two points on the graph was the cyclist moving at the fastest speed?
..................................................................................................................................... (1)
(ii) State one way cyclists can reduce the air resistance acting on them.
..................................................................................................................................... (1)
(iii) How long did the cyclist stop and rest?
..................................................................................................................................... (1)
(iv) Write down the equation which links distance, speed and time.
..................................................................................................................................... (1)
(v) Calculate, in km/hr, the average speed of the cyclist while moving.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Average speed = .............................. km/hr (3)
(Total 7 marks)
Q12. (a) The diagram shows the horizontal forces that act on a moving motorbike.
(i) Describe the movement of the motorbike when force A equals force B.
...........................................................................................................................
........................................................................................................................... (2)
(ii) What happens to the speed of the motorbike if force B becomes smaller than force A?
........................................................................................................................... (1)
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(b) The graph shows how the velocity of a motorbike changes when it is travelling along a straight road.
(i) What was the change in velocity of the motorbike in the first 5 seconds?
........................................................................................................................... (1)
(ii) Write down the equation which links acceleration, change in velocity and time taken.
........................................................................................................................... (1)
(iii) Calculate the acceleration of the motorbike during the first 5 seconds. Show clearly how you work out your answer and give the unit.
...........................................................................................................................
...........................................................................................................................
Acceleration = ............................................. (3)
(c) A car is travelling on an icy road.
Describe and explain what might happen to the car when the brakes are applied.
.....................................................................................................................................
.....................................................................................................................................
..................................................................................................................................... (2)
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(d) Name three factors, other than weather conditions, which would increase the overall stopping distance of a vehicle.
1 ..................................................................................................................................
.....................................................................................................................................
2 ..................................................................................................................................
.....................................................................................................................................
3 ..................................................................................................................................
..................................................................................................................................... (3)
(Total 13 marks)
Q13. The graph shows the speed of a runner during an indoor 60 metres race.
(a) Choose words from this list to complete the sentences below.
moving at a steady speed slowing down
speeding up stopped
Part A of the graph shows that the runner is .............................................................
Part B of the graph shows that the runner is .............................................................
Part C of the graph shows that the runner is .............................................................
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(b) Calculate the acceleration of the runner during the first four seconds. (Show your working.)
....................................................................................................................................
....................................................................................................................................
.................................................................................................................................... (3)
(Total 6 marks)
Q14. The distance-time graph represents the motion of a car during a race.
(a) Describe the motion of the car between point A and point D. You should not carry out any calculations.
To gain full marks in this question you should write your ideas in good English. Put them into a sensible order and use the correct scientific words.
.....................................................................................................................................
..................................................................................................................................... (3)
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(b) Calculate the gradient of the graph between point B and point C. Show clearly how you get your answer.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
gradient = ........................................................................ (3)
(Total 6 marks)
Q15. The graphs in List A show how the velocities of three vehicles change with time. The statements in List B describe different motions.
Draw one line from each graph in List A to the description of the motion represented by that graph in List B.
List A List B Velocity–time graphs Descriptions of motion
(Total 3 marks)
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Q16. A horse and rider take part in a long distance race. The graph shows how far the horse and rider travel during the race.
(a) What was the distance of the race?
distance = .................................................................. km (1)
(b) How long did it take the horse and rider to complete the race?
..................................................................................................................................... (1)
(c) What distance did the horse and rider travel in the first 2 hours of the race?
distance = .................................................................. km (1)
(d) How long did the horse and rider stop and rest during the race?
..................................................................................................................................... (1)
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(e) Not counting the time it was resting, between which two points was the horse moving the slowest?
................................. and ..................................
Give a reason for your answer.
.....................................................................................................................................
..................................................................................................................................... (2)
(Total 6 marks)
Q17. (a) The diagram shows an athlete at the start of a race. The race is along a straight track.
In the first 2 seconds, the athlete accelerates constantly and reaches a speed of 9 m/s.
(i) Use the equation in the box to calculate the acceleration of the athlete.
Show clearly how you work out your answer.
...........................................................................................................................
...........................................................................................................................
Acceleration = .............................. (2)
(ii) Which one of the following is the unit for acceleration?
Draw a ring around your answer.
J/s m/s m/s2 Nm (1)
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(iii) Complete the following sentence.
The velocity of the athlete is the .................................................................... of the
athlete in a given direction. (1)
(iv) Complete the graph to show how the velocity of the athlete changes during the first 2 seconds of the race.
(2)
(b) Many running shoes have a cushioning system. This reduces the impact force on the athlete as the heel of the running shoe hits the ground.
The bar chart shows the maximum impact force for three different makes of running shoe used on three different types of surface.
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(i) Which one of the three makes of running shoe, A, B or C, has the best cushioning system?
...........................................................................................................................
Explain the reason for your answer.
...........................................................................................................................
...........................................................................................................................
........................................................................................................................... (3)
(ii) The data needed to draw the bar chart was obtained using a robotic athlete fitted with electronic sensors.
Why is this data likely to be more reliable than data obtained using human athletes?
...........................................................................................................................
........................................................................................................................... (1)
(Total 10 marks)
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Energy: units and equations
Units
All energy is measured in joules (J)
1 joule is the energy required to lift 1 kg 1m.
Forces are measured in newtons (N)
Power is measured in watts (W)
1 watt is the same as 1 joule per second (J/s or J s-1)