Car Safety

Momentum and Safety answers

A car has a mass of 500 kg. It is travelling at 20m/s. Calculate its momentum.

P = mv = 500 x 20 = 10 000 kgm/s

The driver applies the brakes and stops the car. What is the change in momentum? 10 000 kgm/s

The car takes 4 seconds to stop. Calculate the force exerted by the brakes.

F = ΔP/t = 10 000/4 = 2500 N

The same car is driven into a wall. It is traveling at 20 m/s. What is its momentum before it hits the wall? 10 000 kgm/s The wall stops the car. What is the change in momentum? 10 000 kgm/s The car comes to a halt in 0.006 s. Calculate the force exerted by the wall on the car.

F = ΔP/t = 10 000/0.006 =  1 666 667N

Compare this with the answer to the previous question. Very much larger

What affect does the time taken to stop have on the force applied to the car? The longer the time to stop the less the force

A human head has a mass of 4.5 kg. Calculate its momentum in a car travelling at 15m/s. P= 4.5 x 15 = 67.5 kgm/s

If the person is not wearing a seat belt and the car what happens to the speed of the head? It will hit the windscreen at 15m/s  What is the speed of the head after it has hit the windscreen? 0m/s What is the change in momentum when the skull hits the car windscreen? 67.5kgm/s

The head stops in 0.002 s. calculate the force exerted on the head.

F = ΔP/t = 67.5/0.002 =  33750 N

If the car had air bags fitted the head is brought to a halt in 0.15s. Calculate the force exerted on the head now. F = ΔP/t = 67.5/0.15 =  450 N

A driver has a mass of 65kg she is not wearing a seat belt. Calculate her momentum in a car travelling at 35m/s. 65 x 35 = 2275 kgm/s

 In a crash her car stops and she hits the steering wheel which stops her in 0.05s. Calculate the force exerted on her by the steering wheel. F = ΔP/t = 2275/0.05 = 45 500 N

Had she been wearing a seat belt she would have come to a halt in 0.2s.

Calculate the force exerted on her body by the seat belt. . F = ΔP/t = 2275/0.2 = 11 375 N