Momentum and Safety
1 The car takes 4 seconds to stop. Calculate the force
exerted by the brakes.
2500 N
2 The same car is driven into a wall. It is traveling at 20
m/s. The car comes to a halt in 0.006 s. Calculate the force exerted by the
wall on the car.
F =
1 666 667N = 1.67MN
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
lower the acceleration the less force exerted
3 A human head has a
mass of 4.5 kg.
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
The head stops in 0.002 s. calculate the force exerted on the head.
F = 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 =
450 N
4 A driver has a mass of 65kg she is not wearing a seat
belt. 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 = 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 = 11 375 N
5 A car has a mass of 750 kg and a velocity of 15m/s. (a)Calculate
its KE. ½ x 750 x 152 = 84375 J (b)
What is the work done by the brakes to stop the car 84375 J The brakes exert a maximum
force of 5600 N. (c) Calculate the distance travelled before the car stops. Work = F x d, so, d = work / F = 84375/ 5600 = 15m(d) Repeat this for a speed of 30m/s KE = ½ x 750 x 302 = 337 500 J Work
= F x d, so, d = work / F = 337 500/ 5600 = 60.3m
