P2.1 Force

http://www.youtube.com/watch?v=Zer5IUrZa2Y

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