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Physics Unit 2a

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P2.1.1 – Resultant Forces

3. Balanced and unbalanced forces
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Reaction
Consider a camel standing on a road. What forces are acting on it?
These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere.
Weight

4. Balanced and unbalanced forces
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Reaction
What would happen if we took the road away?
Weight

5. Air Resistance
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Air resistance is a force that opposes motion through air. The quicker you travel, the bigger the air resistance:
The same applies to a body falling through a liquid (called “drag” or “upthrust”).

6. Balanced and unbalanced forces
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7. Balanced and unbalanced forces
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1) This animal is either ________ or moving with _______ _____…
2) This animal is getting ________…
3) This animal is getting _______….
4) This animal is also either _______ or moving with ________ ______..
Words - Stationary, faster, slower or constant speed?

8. Summary Complete these sentences…
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Complete these sentences…
If an object is stationary and has NO resultant force on it the object will…
If an object is stationary and a resultant force acts on it the object will…
If an object is already moving and NO resultant force acts on it the object will…
If an object is already moving and a resultant force acts on it the object will…
…accelerate in the direction of the resultant force
…continue to move at the same speed and the same direction
…continue to stay stationary
…accelerate in the direction of the resultant force

9. Resultant Force Calculate the resultant force of the following: 500N
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Calculate the resultant force of the following:
500N
100N
700N
600N
50N
700N
700N
800N
800N
200N
100N

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P2.1.2 – Forces and Motion

11. Force and acceleration
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If the forces acting on an object are unbalanced then the object will accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2) F A M

12. Force, mass and acceleration
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A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate?
A force of 3000N acts on a car to make it accelerate by 1.5m/s2. How heavy is the car?
A car accelerates at a rate of 5m/s2. If it has a mass of 500kg how much driving force is the engine applying?
A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by? F A M
2m/s2
2000kg
2500N
0.5m/s2

13. Distance, Speed and Time
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Distance, Speed and Time D T S
Speed = distance (in metres)
time (in seconds)
Freddie walks 200 metres in 40 seconds. What is his speed?
Hayley covers 2km in 1,000 seconds. What is her speed?
How long would it take Lauren to run 100 metres if she runs at 10m/s?
Jake travels at 50m/s for 20s. How far does he go?
Izzy drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?
5m/s
2m/s
10s
1000m
500s

14. Distance, Speed and Time
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Distance, Speed and Time D t S
Speed = distance (in metres)
time (in seconds)
Sarah walks 2000m in 50 minutes. What is her speed in m/s?
Jack tries to walk the same distance at a speed of 5m/s. How long does he take?
James drives at 60mph (about 100km/h) for 3 hours. How far has he gone?
The speed of sound in air is 330m/s. Molly shouts at a mountain and hears the echo 3 seconds later. How far away is the mountain? (Careful!)
0.67m/s
400s
300km
495m

15. Distance-time graphs 2) Horizontal line = 40 30 20 10
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2) Horizontal line = 40 30 20 10
4) Diagonal line downwards =
Distance
(metres)
3) Steeper diagonal line =
Time/s
Diagonal line =

16. 40 30 20 10 20 40 60 80 100 0.5m/s 40m 1m/s 40-60s Distance (metres)
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Distance
(metres)
Time/s
What is the speed during the first 20 seconds?
How far is the object from the start after 60 seconds?
What is the speed during the last 40 seconds?
When was the object travelling the fastest?
0.5m/s
40m
1m/s
40-60s

17. Speed vs. Velocity Speed is simply how fast you are travelling…
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Speed is simply how fast you are travelling…
This car is travelling at a speed of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a velocity of 20m/s east

18. Circular Motion 1) Is this car travelling at constant speed?
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1) Is this car travelling at constant speed?
2) Is this car travelling at constant velocity?

19. Acceleration V-U T A Acceleration = change in velocity (in m/s)
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V-U T A
Acceleration
Acceleration = change in velocity (in m/s)
(in m/s2) time taken (in s)
A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her acceleration?
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?
A car accelerates from 10 to 20m/s with an acceleration of 2m/s2. How long did this take?
A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds. What is its acceleration?
2m/s2
120m/s 5s
2000m/s2

20. 40 30 20 10 20 40 60 80 100 1.5m/s 0.5m/s 1m/s Distance (metres)
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Distance
(metres)
Time/s
What was the velocity in the first 20 seconds?
What was the velocity between 20 and 40 seconds?
When was this person travelling the fastest?
What was the average speed for the first 40 seconds?
1.5m/s
0.5m/s
80-100s
1m/s

21. Acceleration V-U T A Acceleration = change in velocity (in m/s)
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V-U T A
Acceleration
Acceleration = change in velocity (in m/s)
(in m/s2) time taken (in s)
Will accelerates from standstill to 50m/s in 25 seconds. What is his acceleration?
Pierre accelerates at 5m/s2 for 5 seconds. He started at 10m/s. What is his new speed?
Elliott is in trouble with the police. He is driving up the A29 and sees a police car and brakes from 50m/s to a standstill. His deceleration was 10m/s2. How long did he brake for?
Another boy racer brakes at the same deceleration but only for 3 seconds. What speed did he slow down to?
2m/s2
35m/s 5s
20m/s

22. Velocity-time graphs 1) Upwards line = 80 60 40 20 4) Downward line =
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1) Upwards line = 80 60 40 20
4) Downward line =
Velocity
m/s
3) Upwards line =
2) Horizontal line =
T/s

23. How fast was the object going after 10 seconds? 80 60 40 20
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Velocity
m/s
T/s
How fast was the object going after 10 seconds?
What is the acceleration from 20 to 30 seconds?
What was the deceleration from 30 to 50s?
How far did the object travel altogether?
40m/s
2m/s2
3m/s2
1700m

24. How fast was the object going after 10 seconds? 80 60 40 20
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Velocity
m/s
T/s
How fast was the object going after 10 seconds?
What is the acceleration from 20 to 30 seconds?
What was the deceleration from 40 to 50s?
How far did the object travel altogether?
10m/s
4m/s2
6m/s2
1500m

25. 80 60 40 20
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Velocity
m/s
T/s
This velocity-time graph shows Coryn’s journey to school. How far away does she live?
2500m

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P2.1.3 – Forces and Braking

27. Stopping a car…
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What two things must the driver of the car do in order to stop in time?

28. Total Stopping Distance = Thinking Distance + Braking Distance
Stopping a car…
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Tiredness
Too much alcohol
Thinking distance
(reaction time)
Too many drugs
Poor visibility
Wet roads
Icy roads
Braking distance
Tyres/brakes worn out
Driving too fast
Total Stopping Distance = Thinking Distance + Braking Distance

29. What happens inside the car when it stops?
Stopping a car…
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What happens inside the car when it stops?
In order to stop this car the brakes must “do work”. This work is used to reduce the kinetic energy of the vehicle and the brakes will warm up.

30. P2.1.4 – Forces and Terminal Velocity
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P2.1.4 – Forces and Terminal Velocity

31. Terminal Velocity Consider a ball falling through a liquid:
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Consider a ball falling through a liquid:
Some questions to consider:
1) What forces are acting on the ball?
2) How do those forces change when the ball gets faster?
3) Will the ball keep getting faster? Explain your answer in terms of forces

32. Words – increase, small, constant, balance, accelerates
Terminal Velocity
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Consider a skydiver:
At the start of his jump the air resistance is _______ so he _______ downwards.
2) As his speed increases his air resistance will _______
3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ________ - this is called TERMINAL VELOCITY
Words – increase, small, constant, balance, accelerates

33. Words – slowing down, decrease, increases, terminal velocity, weight
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Consider a skydiver:
4) When he opens his parachute the air resistance suddenly ________, causing him to start _____ ____.
5) Because he is slowing down his air resistance will _______ again until it balances his _________. The skydiver has now reached a new, lower ________ _______.
Words – slowing down, decrease, increases, terminal velocity, weight

34. Velocity-time graph for terminal velocity…
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Parachute opens – diver slows down
Velocity
Speed increases…
Terminal velocity reached…
On the Moon
Diver hits the ground
New, lower terminal velocity reached
Time

35. Weight vs. Mass
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Earth’s Gravitational Field Strength is 10N/kg. In other words, a 1kg mass is pulled downwards by a force of 10N. W g M
Weight = Mass x Gravitational Field Strength
(in N) (in kg) (in N/kg)
What is the weight on Earth of a book with mass 2kg?
What is the weight on Earth of an apple with mass 100g?
Lee weighs 700N on the Earth. What is his mass?
On the moon the gravitational field strength is 1.6N/kg. What will Lee weigh if he stands on the moon?
20N 1N
70kg
112N

36. P2.1.5 – Forces and Elasticity
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P2.1.5 – Forces and Elasticity

37. Force and Extension Consider a mass on a spring:
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Consider a mass on a spring:
What happens when a mass is added?
When a force is applied to this spring it will change shape and extend. The spring will have “stored elastic potential energy”

38. Elastic Potential Energy
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Elastic potential energy is the energy stored in a system when work is done to change its shape, e.g:
Describe the energy changes when the mass is:
At the top of it’s movement
In the middle
At the bottom

39. Force = Spring constant x extension
Force and Extension
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Task: Find an expression that relates extension to the amount of weight added.
Weight added (N)
Extension (mm) 1 2 3 4 5 6
Force = Spring constant x extension
F = ke

40. Force-Extension Graph for a spring
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Force/N
Extension/mm
The “limit of proportionality”
Force is proportional to extension as long as you don’t go past the “limit of proportionality”

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P2.2.1 – Forces and Energy

42. Work done = Force x distance moved
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When any object is moved around work will need to be done on it to get it to move (obviously).
We can work out the amount of work done in moving an object using the formula:
Work done = Force x distance moved
in J in N in m W D F

43. Example questions 25J 20J, GPE 4m, KE 50N 2MJ
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Hannah pushes a book 5m along the table with a force of 5N. She gets tired and decides to call it a day. How much work did he do?
Courtney lifts a laptop 2m into the air with a force of 10N. How much work does she do? What type of energy did the book gain?
Tom does 200J of work by pushing a wheelbarrow with a force of 50N. How far did he push it? What type of energy did the wheelbarrow gain?
Dan cuddles his cat and lifts it 1.5m in the air. If he did 75J of work how much force did he use?
Simon drives his car 1000m. If the engine was producing a driving force of 2000N how much work did the car do?
25J
20J, GPE
4m, KE
50N
2MJ

44. A Practical Example of Doing Work
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Consider a rocket re-entering the Earth’s atmosphere:
The rocket would initially have a very high _______ energy. This energy would then _____ due to friction caused by collisions with _______ in the atmosphere. These collisions would cause the rocket to ____ up (_____ is “being done” on the rocket). To help deal with this, rockets have special materials that are designed to lose heat quickly.
Words – work, kinetic, particles, heat, decrease

45. Recap questions on Weight
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Jack weighs 600N on the Earth. What is his mass in kg?
Mike pushes Alistair with a force of 20N. If Alistair moves 2m how much work did Mike do on him?
Josh weighs 120N on the moon, where g=1.6N/Kg. What is his mass and what would he weigh on the Earth?
Rose does 100J of work by pushing her pencil case across the table. If she applied a force of 5N how far did she push it?
60kg
40J
75kg, 750N
20m

46. In other words, 1 Watt = 1 Joule per second
Energy and Power
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The POWER RATING of an appliance is simply how much energy it uses every second.
In other words, 1 Watt = 1 Joule per second E T P
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)

47. Some example questions
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What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?
What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?
Rob runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating?
How much energy does a 150W light bulb transfer in a) one second, b) one minute?
Jonny’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute physics lesson?
Lloyd’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?
60W
2KW
0.2MW
150J, 9KJ
120KJ
1.73MJ

48. Gravitational Potential Energy
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To work out how much gravitational potential energy (GPE) an object gains when it is lifted up we would use the simple equation…
GPE = Weight x Change in height
(Joules) (newtons) (metres)
GPE H mg
(Remember - W=mg)

49. Some example questions…
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How much gravitational potential energy have the following objects gained?:
A brick that weighs 10N lifted to the top of a house (10m),
A 1,000kg car lifted by a ramp up to a height of 2m,
A 70kg person lifted up 50cm by a friend.
How much GPE have the following objects lost?:
A 2N football dropping out of the air after being kicked up 30m,
A 0.5N egg falling 10m out of a bird nest,
A 1,000kg car falling off its 200cm ramp.
100J
20KJ
350J
60J 5J
20KJ

50. Kinetic energy = ½ x mass x velocity squared
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Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using the formula:
Kinetic energy = ½ x mass x velocity squared
in J in kg in m/s
KE = ½ mv2

51. Example questions
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Bex drives her car at a speed of 30m/s. If the combined mass of her and the car is 1000kg what is her kinetic energy?
Emma rides her bike at a speed of 10m/s. If the combined mass of Emma and her bike is 80kg what is her kinetic energy?
Rob is running and has a kinetic energy of 750J. If his mass is 60kg how fast is he running?
Josh is walking to town. If he has a kinetic energy of 150J and he’s walking at a pace of 2m/s what is his mass?
450,000J
4000J
5m/s
75kg

52. Test questions… 8.3m/s 16m/s 0.5J 120N 50s 2m/s2 20m/s 375N
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Julia tries to run 100m in 12 seconds and succeeds. How fast did she run?
Isabelle accelerates at a rate of 2m/s2 for 3 seconds. If she started at 10m/s what was her final speed?
Jake decides to lift his book up into the air. His book has a mass of 100g and he lifts it 50cm. Calculate the work done.
Jamie accelerates from 0 to 10m/s in 5 seconds. If her mass is 60kg how much force did her legs apply?
Lily rides 1km at a speed of 20m/s. How long did the journey take?
Rob thinks it’s funny to push Jack with a force of 140N. If Jack has a mass of 70kg calculate his acceleration.
Vicky slams on the brakes on her bike and her brakes do 20,000J of work. If the combined mass is 100kg what speed was she travelling at?
Paddy has a mass of 75kg. If he accelerates from 10 to 20m/s in 2s how much force did he apply?
8.3m/s
16m/s
0.5J
120N
50s
2m/s2
20m/s
375N

53. Test questions… 0.25s 100m/s2 700N 5m 70kg 5m/s 875J
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Bex amuses herself by throwing things at Kit. If she throws a ball with a speed of 20m/s and the distance between her and Kit is 5m how long will it take to reach him?
Dave throws calculators around the room with a force of 20N. If each calculator has a mass of 200g calculate the acceleration.
Max has a mass of 70kg. What is his weight on Earth, where the gravitational field strength is 10N/kg?
Kathryn does some work by pushing a box around with a force of 1N. She does 5J of work and decides to call it a day. How far did she push it?
On the moon Jake might weigh 112N. If the gravitational field strength on the moon is 1.6N/kg what is his mass?
Heather likes bird watching. She sees a bird fly 100m in 20s. How fast was it flying?
How much kinetic energy would Simon have if he travelled at a speed of 5m/s and has a mass of 70kg?
0.25s
100m/s2
700N 5m
70kg
5m/s
875J

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P2.2.2 – Momentum

55. Momentum
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Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: P V M
Momentum = Mass x Velocity
(in kgm/s) (in kg) (in m/s)
What is the momentum of the following?
A 1kg football travelling at 10m/s
A 1000kg Ford Capri travelling at 30m/s
A 20g pen being thrown across the room at 5m/s
A 70kg bungi-jumper falling at 40m/s
10kgm/s
30,000kgm/s
0.1kgm/s
2800kgm/s

56. Conservation of Momentum
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In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question:
Two cars are racing around the M25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision?
Speed = 50m/s
Speed = 20m/s
Mass = 1000kg
Mass = 800kg
Speed = ??m/s
Mass = 1800kg
Momentum before = momentum after…
…so 1000 x x 20 = 1800 x V…
…V = 36.7m/s

57. Momentum in different directions
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What happens if the bodies are moving in opposite directions?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Momentum is a VECTOR quantity, so the momentum of the second car is negative…
Total momentum = 1000 x 50 – 800 x 20 = kgm/s
Speed after collision = kgm/s / 1800 = 18.9m/s

58. Another example
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Consider the nuclear decay of Americium-241: α 4 2 Np
237 93 Am
241 95
If the new neptunium atom moves away at a speed of 5x105 m/s what was the speed of the alpha particle?
2.96x107 m/s

59. More questions…
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A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at?
A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving?
A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have a mass of 1kg. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball?
A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at?
5.6m/s
70m/s
3m/s
400m/s

60. Recap question on momentum
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Matt and Dan are racing against each other over 400m at Sports Day. Matt is running at 8m/s and catches up with Dan who is running at 6m/s. After the collision Matt stops and Dan moves slightly faster. If Matt’s mass is 60kg and Dan’s is 70kg calculate how fast Dan moves after the collision.
Bobbie is driving her 5kg toy car around. It is travelling at 10m/s when it hits the back of Heather’s (stationary) leg and sticks to it. Assuming Heather’s leg can move freely and has a mass of 10kg calculate how fast it will move after the collision.
12.9m/s
3.3m/s

61. Safety features How do air bags and crumple zones work? Basically:
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How do air bags and crumple zones work?
Basically:
The change in momentum is the same with or without an airbag
But having an airbag increases the time of the collision
Therefore the force is reduced