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Car Physics: Crashes & Traction

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Car Physics: Crashes & Traction

Investigate how momentum, crash protection and tyre grip affect a car’s motion. Use the information on this sheet. For calculations, use momentum = mass × velocity and stopping distance = velocity² ÷ (2 × deceleration). Convert km/h to m/s by dividing by 3.6. Include units in your answers.

Crash investigation

A road-safety team compares two test cars. Both cars travel in a straight line before braking. In a controlled test, a car’s momentum is conserved during the very short collision if outside forces are negligible. Kinetic energy may change into sound, heat and deformation. Seatbelts and airbags increase the time over which a person slows down; crumple zones deform to absorb energy.

1.A 1,200 kg car travels at 15 m/s. Calculate its momentum. Show the formula, substitution and unit.

2.In the test described above, two cars lock together after colliding. Which quantity is conserved in the collision when outside forces are negligible?

The total momentum of the two-car system
The total kinetic energy in every collision
The speed of each car
The force on each car

3.Explain why a crumple zone can reduce the force on a passenger during a crash. Use the idea of energy transfer or stopping time.

4.Select every feature that can reduce injury severity once a crash occurs.

Seatbelt
Airbag
Crumple zone
Tyres with deeper tread

Traction and stopping

Traction is the grip between a tyre and the road. Braking distance is the distance travelled after the brakes are applied; it is not the same as total stopping distance, which also includes thinking distance. In a simplified calculation, assume constant deceleration and ignore thinking distance. Tread grooves channel water away from the tyre’s contact area, helping maintain contact with the road. On wet roads, drivers can reduce crash risk by slowing down and leaving a larger following distance.

5.Choose the conditions that are likely to increase a car’s braking distance.

Higher starting speed
Wet or loose road surface
Worn or poorly inflated tyres
A brightly coloured car

6.A car is travelling at 72 km/h. Convert its speed to metres per second. Show your calculation.

7.The car travelling at 72 km/h slows with an average deceleration of 5 m/s². Estimate its braking distance using s = v² ÷ (2a). Use your answer to the previous question for v.

8.On the same road and with the same deceleration, a car’s speed doubles. What happens to its braking distance according to s = v² ÷ (2a)?

It doubles
It becomes four times as long
It is halved
It stays the same

9.A tyre has grooves in its tread. Using the information above, explain one way grooves can help maintain traction on a wet road, and describe one way a driver can reduce crash risk in wet conditions.