17 March 2016
Change in momentum
The Crumple Zone Challenge
Objectives
To be able to use the change in momentum to help
understand car safety features.
HSW: AF4 – Using investigative approaches
Used before in: Science A
Will use again in:
PLTS: Self-managers – organise own time.
Used before in: Science A
Will use again in:
Keywords
mass, velocity, m/s, kg, scalar, vector, momentum, kgm/s,
inertia, collision, conservation, kinetic energy, crumple zone
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Outcomes
• ALL SHOULD be able to carry out an
investigation into crumple zones and
change in momentum safely.
• MOST SHOULD be able to describe
momentum changes in collisions.
• SOME COULD use equations to work out
momentum change and impact force in
collisions.
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Force and change in momentum
When a force is applied to an object, the object’s velocity
changes. This means that its momentum will also change.
The change in momentum depends on the size of the
force and the time for which it is applied. The relationship
between this values is shown by this equation:
force = change in momentum
time
 Momentum is measured in kilogram metres per
second (kg m/s).
 Time is measured in seconds (s).
 Force is measured in newtons (N).
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Change in momentum question 1
A rugby ball of mass 0.5 kg is
kicked from stationary to a velocity
of 8 m/s. The kicker’s foot is in
contact with ball for 0.1 seconds.
What force does the kicker use?
change in momentum
force =
time
= (0.5 x 8) – ( 0.5 x 0)
0.1
= 4
0.1
= 40 N
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Change in momentum question 2
A tennis ball is rolled at a toy car of mass 0.1 kg. The car
is moved with a velocity of 0.5 m/s. If the ball and car are
in contact for 0.05 seconds, with what force is the tennis
ball is rolled?
change in momentum
force =
time
= (0.1 x 0.5) – ( 0.1 x 0)
0.1
= 0.05
0.05
= 1N
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Car crashes and momentum
What happens if two cars travelling very quickly collide?
Both cars come to a stop
in a short space of time.
This means that the cars
and their occupants
experience a large change
of momentum very quickly.
Why could this cause a
very serious injury?
A very large change of momentum in a short space of time
means the car occupants will experience a very large force.
Using this principle, how could you improve the safety of
cars?
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Reducing force in car crashes
Many modern car safety features work by increasing the
amount of time taken for the person to decelerate in a
collision. How does this reduce the risk of serious injury?
A longer deceleration means that change in momentum
occurs over a longer time. There is therefore a smaller
force acting on the person.
What features of cars
use this principle?
 seatbelts
 airbags
 crumple zones
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How do car safety features work?
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Making a crumple zone
• Task:
• In groups, make a crumple zone from 2 A4
pieces of a paper and a length of sellotape
provided by your teacher.
• You have 20 minutes to make it and then it
will be testing time
• Whichever trolley records the smallest force
on the force metre will win house points!!
• Remember – larger time, smaller impact.
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Results
Team Name
Impact Force /Newtons
Analysis: Look at the winning design.
Can you explain why it won?
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Homework
• Revise for test on Kinetic Energy and
Momentum.
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Change in momentum calculations
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