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Change in momentumThe Crumple Zone Challenge

19 April 2023

Keywords

mass, velocity, m/s, kg, scalar, vector, momentum, kgm/s, inertia, collision, conservation, kinetic energy, crumple zone

ObjectivesTo 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:

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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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When a force is applied to an object, the object’s velocity changes. This means that its momentum will also change.

Force and change in momentum

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:

Time is measured in seconds (s).

Momentum is measured in kilogram metres per second (kg m/s).

Force is measured in newtons (N).

force = change in momentum

time

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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 question 1

= 40 N

force =change in momentum

time

= (0.5 x 8) – ( 0.5 x 0)

0.1

= 4

0.1

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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 question 2

= 1 N

force =change in momentum

time

= (0.1 x 0.5) – ( 0.1 x 0)

0.1

= 0.05

0.05

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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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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?

Reducing force in car crashes

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