© boardworks ltd 2004 1 of 20 © boardworks ltd 2005 1 of 32 ks4 physics speed and acceleration

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 32

KS4 Physics

Speed and Acceleration


Contents


Stopping distance

Plotting the speed / time graph

Summary activities

Formula triangles


Stopping distance is the sum of the thinking distance and the braking distance.

Stopping distances

Thinking distance is the distance travelled before the brakes are applied.

Braking distance is the distance travelled whilst the brakes are being applied.

Stopping distance = thinking distance + braking distance

How long does it take a moving vehicle to stop?


Definitions

Stopping distance

Friction

Thinking distance

Braking distance

One of forces the road exerts on the tyres as the car is stopping.

The distance a car travels whilst it is braking.

The distance a car travels before the brakes are applied.

The sum of thinking distance and the braking distance.


What affects braking/thinking distance?

Thinking distance Braking distance

Speed of car

Speed of car Speed of car

Road conditions

Road conditions

Drugs and alcohol

Drugs and alcohol

Tiredness

Tiredness

Medication

Medication

Condition of tyres

Condition of tyres

Condition of brakes

Condition of brakes


Braking car questions

a) What do we call the distance the car travels before the driver applies the brakes?

b) Name one factor that could increase the distance the car travels in this time.

c) The braking distance for the car is 35m. If the stopping distance is 50m, how far did the car travel before the driver applied the brakes?

Thinking distance

Medication, drugs/alcohol, speed of car, tiredness

Thinking distance = Stopping distance – braking distance

= 15m= 50m – 35m

A car is moving along an open road. Suddenly, a sheep walks into the road.


This graphing experiment shows an animation of a car travelling along a straight road.

1. Copy the results table shown on the next slide and complete it as the movie is played.

2. Record the distance the car has travelled every five seconds.

3. Plot a graph of your results.

Car graphing activity – instructions


Results table for distance/time graph

Time/seconds Distance/metres

0

5

10

15

20

25

30

35

40

45

50

55

Car graphing activity – results table layout


Car graphing activity – animation


Results table for distance/time graph

Time/seconds Distance/metres

0 0

5 16

10 76

15 186

20 234

25 484

30 634

35 784

40 904

45 974

50 994

55 994

Car graphing activity – results Car graphing activity – results


Distance / Time graph for car

0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce /

met

res

Car graphing activity – results graph



0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce /

met

res

The car has stopped. The graph is flat – the distance of the car from the start point is not changing.The graph is straight – there is no change in speed.

The speed of the car is changing – the graph is not flat. The slope of the graph is less steep as the car begins to slow down.

The car is starting to move. The curve shows that the speed is changing. The curve is upwards as the car accelerates at the start of the journey.

The car is going fast but at a constant speed.The graph is straight in this part of the journey.

Car graphing activity – results graph analysis


The speed of the car can be calculated by looking at the gradient of the distance/time graph.

Speed is “Distance Travelled divided by Time Taken”

These values can be read off the distance/time graph at different points, and this is the same as the gradient of the graph.

Gradient of a distance/time graph



0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce

/ m

etr

es

Consider the gradient of this graph at the point shown by the two arrows in a triangle:

The car has travelled from 200m to 800m = 600m.It took from 16s to 36s to travel this distance = 20s.

Gradient of a distance/time graph

So the speed at this point = 600m/20s = 30m/s.


Contents


Stopping distance


Summary activities

Formula triangles


1. Copy the results table shown on the next slide and complete it as the movie is played

2. Record the speed of the car at five second intervals.

3. Then graph your results.


Having looked at the distance-time graph, plot the speed-time graph:


Results table for speed/time graph

Time/seconds Speed (m/sec)

0

5

10

15

20

25

30

35

40

45

50

55

Car graphing activity – results table layout


Car activity – animation


Car graphing activity – results

Results table for speed/time graph

Time/seconds Speed (m/sec)

0 0

5 6

10 16

15 26

20 30

25 30

30 30

35 30

40 20

45 10

50 0

55 0


Speed / Time graph for car

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Me

tre

s/s

ec

on

d

Care accelerating –speed is increasing.

Car at constant speed –acceleration is zero.

Speed / time graph for a car

Car decelerating –speed is decreasing.

Car at rest – zero speed


Now compare the Speed /

Time graph with the earlier

Distance / Time graph

Speed / Time graph for car

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Me

tre

s/s

ec

on

d


0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce

/ m

etr

es

The speed is decreasing

and the curve is downwards

Speed / time graph for a car

The speed is zero – the car is not

moving – and we can see that the distance that the car has travelled is not

changing either.

From both graphs we

can see that the speed is

30 m/s.

(Using the value

calculated previously)

The speed is increasing, and we

can see that the Distance / Time graph

curves upwards.


Contents


Stopping distance


Summary activities

Formula triangles


Calculating speed

We can express the speed formula using the equation:

speed = distance ÷ time

s = d/t

Speed measured in metres per second (m/s)

Distance measured in metres (m)

Time measured in seconds (s)


Speed formula triangle

d

s t

x

Formula triangles help you to rearrange formula. The triangle for the speed formula is shown below.

Cover up whatever quantity you are trying to find, and you will be left with the calculation required.

1) So if you were trying to find speed (s)…

2) …you would cover up s…

3) …and you are left with the sum…

s = d ÷ t


Interactive formula triangle


Speed of vehicles

100 m

Measure out a known distance, say 100m, alongside a road.

Record the time it takes vehicles to cover the distance.

Use the speed formula, s=d/t, to calculate the speeds of various vehicles.

Measure the speed of at least 20 vehicles and then represent your data graphically.


Calculating acceleration

We can express the formula for acceleration using the equation:

acceleration = change in velocity ÷ time taken

a = c/t

Acceleration is measured inmetres per second per second (m/s2)

Change in velocity is measured inmetres per second (m/s)

Time measured is in seconds (s)


c

a t

x

1) So if you were trying to find acceleration (a)...

2) …you would cover up a…

3) …and you are left with the sum…

a = c ÷ t

Formula triangles help you to rearrange formula. The triangle for the acceleration formula is shown below.

Cover up whatever quantity you are trying to find, and you will be left with the calculation required.

Acceleration formula triangle


Contents


Stopping distance


Summary activities

Formula triangles


acceleration – The rate of change of velocity per unit time. It is measured in metres per second squared (m/s2).

braking distance – The distance a car travels while the brakes are being applied.

friction – The force that tries to stop materials moving over each other. It occurs between a road surface and car tyres.

speed – How fast an object is moving. It equals the distance moved divided by the time taken and is usually measured in metres per second (m/s).

stopping distance – The total distance needed to stop a car. It is the thinking distance plus the braking distance.

thinking distance – The distance a car travels while the driver is thinking before the brakes are applied.

velocity – The speed at which an object is travelling in a particular direction. It is measured in metres per second (m/s).

Glossary


Anagrams


Multiple-choice quiz

© boardworks ltd 2004 1 of 20 © boardworks ltd 2005 1 of 32 ks4 physics speed and acceleration

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