p2.1 force
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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.
Candidates should use their skills, knowledge and understanding 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
Overview
P2.1.1 Force - Objectives
• Find out what happens when two objects interact
• Investigate the ‘resultant’ force on an object
• State what happens when a resultant force acts on an object
This is an object This is the force
This is an object This is the force
Weight
Lift
DragThrust
Weight
Lift
Drag Thrust
3N 5N 2N
96N 96N 0N
3N 8N 5N
3N
4N5N
EXTENSION
2
1
3
4
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.
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.
Weight
Reaction
Weight
Reaction
DragThrust
P2.1.2 Force and Motion - Objectives
• State what happens when a resultant force acts on an object
• Investigate what affects the acceleration of an object
• Use F = m x a in calculations
3N 8N
5N
The object will accelerate in the direction of the resultant force.
Resultant Force =
Mass x AccelerationResultant Force =[N] [kg] [m/s2]
Q1. The vehicle has a mass of 1200kg. The engine is applying a forward force of 500N and the friction from the road is a force of 70N.
a) What is the acceleration of the vehicle?
(show your working)
Total forward force = 500 – 70 = 430N
Mass = 1200kg
F = ma
a = F/m = 430/1200 = 0.36 m/s²
P2.1.2 Force and Motion - Objectives
• Read a velocity-time graph• Use a graph to perform calculations• Recognise the difference between speed
and velocity• Calculate the acceleration of an object
P2.1.2 Force and Motion
1. Finish your table, calculating the speed (velocity) of the paper.
2. Plot a graph on A4 graph paper:– Velocity (in m/s) on vertical (y) axis– Time (in s) on horizontal (x) axis
3. What does this graph show?
This is not an easy task – do not be afraid of getting it wrong
Velocity-time graph• A mountain biker accelerates uniformly down a
ramp taking 10s to reach 20m/s.• They stay at 20m/s for the next 30s of the course.• They then take 5s to slow down to 10m/s as they
enter the technical, steep part of the track. They stay at 10m/s for the next 20 seconds.
• Breaking out of the woods they accelerate to 30m/s in 5s as they hit the high speed part of the course. They maintain 30m/s for the next 20s.
• They cross the finish line and it takes them 5s to come to a stop.
http://www.youtube.com/watch?v=LSqj8c8LaBQ&feature=related
Time(s)
Start velocity(m/s)
Final velocity(m/s)
Change in Velocity
(m/s)Acceleration
(m/s2)Distance
Travelled (m)
0-10 0 20 20 2
Time(s)
Start velocity(m/s)
Final velocity(m/s)
Change in Velocity
(m/s)Acceleration
(m/s2)Distance
Travelled (m)
0-10 0 20 20 2 100
10-40 20 20 0 0 600
40-45 20 10 -10 -2 75
45-65 10 10 0 0 200
65-70 10 30 20 4 100
70-90 30 30 0 0 600
90-95 30 0 -30 -6 75
Time(s)
Start velocity(m/s)
Final velocity(m/s)
Change in Velocity
(m/s)Acceleration
(m/s2)Distance
Travelled (m)
0-10 0 20 20 2
10-40 20 20 0 0
40-45 20 10 -10 -2
45-65 10 10 0 0
65-70 10 30 20 4
70-90 30 30 0 0
90-95 30 0 -30 -6
Answer all Summary Questions in Textbook:Pages 109, 111 and 113
Complete ‘Speed and acceleration 2’ worksheet
Sort out your book• Stick in sheets• Complete unfinished
work
Past exam questions:1, 2 and 3 on page 117
All these tasks need to be completed at some point
Stopping Distance=
Thinking Distance + Braking Distance
• Drinking• Drugs• Tiredness• Distractions
• Icy road• Wet road• Poor brakes• Worn tyres
Stopping Distance=
Thinking Distance + Braking Distance
Spee
d
Stopping Distance=
Thinking Distance + Braking Distance
Spee
d
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 increase.
P2.1.3 Forces and Braking
Kinetic Energy Heat Energy
P2.1.4 Forces and Terminal Velocity
http://www.youtube.com/watch?v=HN-U_INtUSs&feature=related
http://www.youtube.com/watch?v=CYTYia9k4Tw
Weight
1
P2.1.4 Forces and Terminal Velocity
F = mgWhen a parachutist
first jumps out of the
plane
Weight
2
P2.1.4 Forces and Terminal Velocity
Drag
F = 0
When they reach
terminal velocity
Weight
3
P2.1.4 Forces and Terminal Velocity
Drag
F = up
When they open their
parachute
Weight
4
P2.1.4 Forces and Terminal Velocity
Drag
F = 0When they descend
at a lower terminal
velocity
Weight
5
P2.1.4 Forces and Terminal Velocity
Reaction
F = up
When they land on
the ground
v-t graph for terminal velocity
Time / seconds
Velo
city
/ m
/s
1
2
3
45
P2.1.5 Force and Elasticity - Objectives
• Realise what happens when a force acts on an object
• Reason why objects stretch and store energy
• Relate stored energy to the work done• Investigate how the extension is related
to the force applied
Mass / kg Weight / N Total Length / m
Extension / m
0 0 00.1 10.2 20.30.40.5
Results table
Force-extension graph
Spring
Force / N
Exte
nsio
n / m
Elastic Band
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