Starter Momentum
 Watch: https://www.youtube.com/watch?v=OiTiKOy59o4
 What happenend with Sandra Bullock?
 What is the action force?
 What is the reaction force?
 Do the action force and the reaction force act on the same
object?
 What can we say about the size of the forces?
 What does Newton’s third law state?
 Watch
https://www.youtube.com/watch?v=CCtN9jwhcyg
 What happenend with the two cars?
 Is Newton’s third law valid? Why? Why not?
 Do both cars have a momentum before the collision?
 Why? Why not?
 Have they got the same momentum?
 What is the equation for momentum?
P
Momentum = Mass x Velocity
(in kgms-1)
(in kg)
(in ms-1)
M
 What can you tell me about the momentum after the
collision?
 How do we calculate the change in momentum?
V
Conservation of
Momentum
Starter
• A car crash is an example of a serious
collision.
• Can you think of some less serious
“everyday” examples?
Learning Objectives
• Know how to reduce injuries in a
collision.
• Know what conservation of
momentum is
Success criteria
• To be able to explain why a collision
can cause injury.
(Grade C)
• To be able to explain how to reduce
injury in a collision.
(Grade C)
• To be able to carry out calculations
related to conservation of
momentum
(Grade A)
Watch the video:
Why would this
have happened?
←
Force = change in momentum
time
Therefore...
• Short impact time = large force
• Large impact time = small force
So, how can we avoid or reduce
injuries in collisions?
P3 revision
Safety
features
in/on a car
Worksheet – stopping more safely
Decreasing the Force
• We need to decrease the force experienced by
passengers in a crash.
• If:
Force = change in momentum
time
... Then the force can be decreased if the time it
takes for the vehicle to stop is increased.
• For example, crumple zones cause energy to be
absorbed over an extended period, decreasing
velocity more gradually. This channels energy away
from, and reduces the force on the people inside
the car.
• Foundation: w/s P5d4 – old course
• Higher tier: work on conservation of
momentum
Conservation of Momentum
In any collision or explosion momentum is conserved (provided that
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 = 50ms-1
Mass =
1000kg
Speed = 20ms-1
Mass = 800kg
Mass =
1800kg
Speed = ??ms-1
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7ms-1
Momentum in different directions
What happens if the bodies are moving in opposite directions?
Speed = 50ms1
Speed = 20ms1
Mass =
1000kg
Mass = 800kg
Momentum is a VECTOR quantity, so the momentum of the second car is
negative…
Total momentum = 1000 x 50 – 800 x 20 = 34000 kgms-1
Speed after collision = 34000 kgms-1 / 1800 = 18.9ms-1
Now do worksheet P5d6 – old
course
Answers
1.
2.
3.
4.
5.
6.
Very rapid acceleration of parts of the body
To extend the time of any collision, reducing the rate of change of
momentum, and so the force on the motorcyclist’s head
The front crumples in an accident, prolonging the time taken to stop; this
reduces the rate of change of momentum, and so the force on the people
in the car; the passenger compartment needs to be strong so that it does
not collapse, injuring the occupants
To prevent the person wearing it stopping too quickly in a collision; this
reduces the rate of change of momentum, and so the force on the person
wearing it, reducing injury
An air bag inflates and squashes; it slows the driver down more slowly as
well as protecting them from protruding objects; this protects both the
head and thorax (chest) of the driver
Gymnasts bend their knees on landing so that they stop more slowly,
reducing the rate of change of momentum, and so the force on their
knees
Higher
1.
3 m/s
2.
20 Ns (or kg m/s)
3.
2.2 m/s
4.
0.0016 s
5.
Total momentum = 0 before firing pellet, so total momentum = 0 after
firing pellet as momentum is conserved; this means ‘momentum of pellet
forwards = momentum of rifle backwards’
–
–
–
0.4 kg m/s
0.4 kg m/s
0.2 m/s
Explosions
• An explosion is the opposite of a
collision – objects move apart.
However, momentum is still
conserved. For example...
• Before firing, total momentum is
zero.
• After firing, total momentum must
also be zero
• As the bullet moves forward, the gun
must move backwards (it recoils)
Learning Objectives
• Know how to reduce injuries in a
collision.
• Know what conservation of
momentum is
Success criteria
• To be able to explain why a collision
can cause injury.
(Grade C)
• To be able to explain how to reduce
injury in a collision.
(Grade C)
• To be able to carry out calculations
related to conservation of
momentum
(Grade A)
Plenary
• Watch the video:
• The car is designed to break up. Why?