Additional Physics – Forces

L/O :- To be able to calculate acceleration

“The need for speed”

Exam Date -

Quick quiz • What can the motion be like of an object

with BALANCED forces? (2 marks) • What can the motion be like of an object

with UNBALANCED forces (2 marks) • How do you work out speed?• What does this distance time graph tell

you about the journey?

These cars all have the same speed but different velocities. Why is

this?

So what is the difference between speed and velocity?

How can you work out velocity?

19/04/23

Speed vs. Velocity

Speed is simply how fast you are travelling…

Velocity is “speed in a given direction”…

This car is travelling at a speed of 20m/s

This car is travelling at a velocity of 20m/s east

These vehicles all have a similar engine size (roughly 300 horse power) Which one will accelerate the fastest and why?

The bike will have the fastest acceleration because it has the lowest

mass You can prove this using the equation

F = m x a

F m x a

1273kg

281kg

11339kg

Use the equation to check your predictions

AccelerationV-U

TA

Acceleration = change in velocity (in m/s)

(in m/s2) time taken (in s)

1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration?

2) A ball is dropped and accelerates downwards at a rate of 10m/s2 for 12 seconds. How much will the ball’s velocity increase by?

3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take?

4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?

Don’t forget to underline the key info in an exam

19/04/23

Velocity-time graphs

80

60

40

20

0 10 20 30 40 50

Velocity

m/s

T/s

1) Upwards line =

2) Horizontal line =

3) Upwards line =

4) Downward line =

80

60

40

20

0

1) How fast was the object going after 10 seconds?

2) What is the acceleration from 20 to 30 seconds?

3) What was the deceleration from 30 to 50s?

4) How far did the object travel altogether?

10 20 30 40 50

Velocity

m/s

T/s

The time starts when the boy gets on the bike. All times must be recorded as how many seconds after this.

IncidentBoy gets on bike

[A]

Leaps bike over

hedge [B]

Bike swerves in front of lorry

[C]

Lorry hits first car [D]

Bike goes down

ramp [E]

Bike stops near

bridge [F]

Speed (m/s)

0 6 6 9 9 0

Time (s)            

The time starts when the boy gets on the bike. All times must be recorded as how many seconds after this.

IncidentBoy gets on

bike [A]

Leaps bike over hedge

[B]

Bike swerves in front of lorry [C]

Lorry hits first car [D]

Bike goes down ramp [E]

Bike stops near bridge

[F]

Speed (m/s) 0 6 6 9 9 0Time (s)            

IncidentWe first see

Terminator [G]

Terminator shoots at gate

chains

Terminator’s bike drops into drain

Lorry’s tyre is burst [H]

Lorry explodes [I]

Speed (m/s) 12 12 12 12 0

Time (s)          

IncidentBoy gets on bike

[A]

Leaps bike over

hedge [B]

Bike swerves in front of lorry

[C]

Lorry hits first car

[D]

Bike goes down

ramp [E]

Bike stops near

bridge [F]

Speed (m/s)

0 6 6 9 9 0

Time (s)            

IncidentWe first see Terminator

[G]

Terminator shoots at

gate chains

Terminator’s bike

drops into drain

Lorry’s tyre is burst [H]

Lorry explodes [I]

Speed (m/s)

12 12 12 12 0

Time (s)          

Task 2

1. Plot the results above on a speed-time graph, with the x-axis labelled time and the y-axis labelled speed. 2. At each point where the boys changes motion should be labelled with the letter A, B, C, … The point (0,0) should be labelled A.

3. Use the shape of your graph to label the point where you expect acceleration to be

a) f astest b) slowest

4. Work out the f ollowing accelerations: A to B B to C C to D D to E

E to F F to G G to H H to I

1. 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..........................................................................................................................................................................................................................................................................Answer .............................................................(2)(b) Calculate the force needed to produce this acceleration................................................................................................................................................................................................................................................................................................................................................................................................................(3)(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................................................................................................................................................................................................................................................................................................................................................................................................................

2. The graph shows the speed of a runner during an indoor 60 metres race.

sp eed(m e tres p e r

seco n d )

tim e (sec on d s)

1 0

5

00 2 4 6 8 1 0

A

B

C

(a) Choose words from this list to complete the sentences below.moving at a steady speed slowing down speeding up stoppedPart 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 .............................................................(3) (b) Calculate the acceleration of the runner during the first four seconds. (Show your working.).................................................................................................................................... (3)

3. (a) The diagram below shows a moving tractor. The forward force from the engine exactly balances the resisting forces on the tractor.

(i) Describe the motion of the tractor..........................................................................................................................(ii) The tractor comes to a drier part of the field where the resisting forces are less. If the forward force from the engine is unchanged how, if at all, will the motion of the tractor be affected?.................................................................................................................................................................................................................................................. (3)

Distance travelled (m)

Time taken (s)0

0

40

8

80

16

120

24

160

32

200

40

D is tan ce(m )

2 0 0

1 8 0

1 6 0

1 4 0

1 2 0

1 0 0

8 0

6 0

4 0

2 0

00 1 0 2 0 3 0 4 0

Tim e (s )

(b) Two pupils are given the task of finding out how fast a tractor moves across a field. As the tractor starts a straight run across the field the pupils time how long it takes to pass a series of posts which are forty metres apart. The results obtained are shown in the table below.

(i) Draw a graph of distance travelled against time taken using the axes on the graph below. Label your graph line A.

(2)(ii) Calculate the speed of the tractor......................................................................

In another, wetter field there is more resistance to the movement of the tractor. It now travels at 4 m/s.(i) Calculate the time needed to travel 200m.............................................................................

(ii) On the graph in part (b) draw a line to represent the motion of the tractor across the 2nd field. Label this line B.(4) (d) On a road the tractor accelerates from rest up to a speed of 6 m/s in 15 seconds.Calculate the acceleration of the tractor......................................................................................................................................................................................................................................................................................................................................................Acceleration = ..................m/s²(3) (Total 15 marks)

4. The graph shows the speed of a runner during an indoor 60 metres race.

S p eed(m e tres p e r

seco n d )

Tim e (seco n d s)

1 0

5

00 2 4 6 8 1 0

(a) Calculate the acceleration of the runner during the first four seconds. (Show your working.)....................................................................................................................................................................................................................................................................................................................................................(b) How far does the runner travel during the first four seconds? (Show your working.)....................................................................................................................................................................................................................................................................................................................................................(c) At the finish, a thick wall of rubber foam slows the runner down at a rate of 25 m/s2. The runner has a mass of 75kg. Calculate the average force of the rubber foam on the runner. (Show your working.)....................................................................................................................................................................................................................................................................................................................................................Answer ......................... newtons (N)

(2)

5. The manufacturer of a family car gave the following information.Mass of car 950 kgThe 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)

(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)

6. A rollercoaster car stops above a vertical drop. Suddenly it falls under gravity.

The drop is 60 metres high and at the bottom of the drop the car travels at 125 km/h.The acceleration experienced by the people in the car is 10 m/s2. The mass of the car and itspassengers is 1210 kg.

Calculate the force exerted on the car and its passengers. Show your working............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................Force = ........................... N

(Total 3 marks)

What have you learnt?

• Do you know what the difference between speed and velocity is?

• Do you know how to work out acceleration?

• Do you know which way the axes on a velocity-time graph go?

• Do you know how to work out acceleration from a velocity-time graph?