Calculating momentum, velocity, impulse and force.

Question

A car (m=900kg) travelling at 70km/h hits a pedestrian (m=60kg) who is travelling at 1m/s in the same direction. After the collision the car has a velocity of 30km/h in the same direction it was originally travelling. The collision took 0.4 seconds to occur and was completely elastic. 

a) Calculate the total initial momentum

b) Calculate the velocity of the pedestrian after the collision

c) Calculate the impulse on the pedestrian

d) Calculate the average force of the car on the pedestrian

Answer 1

First, you have to have consistency in units

km/h = 1000/3600 m/s

momentum = mass * velocity

a) Total Initial momentum = 900*70*1000/3600 + 60 * 1 

= 17,560 kg.m/s

b) momentum is conserved 17,560 = 900*30*1000/3600 + 60v; v = 168 m/s

c) impulse = change of momentum = 60(168-1) =10,020 m.kg/s

d) average force = rate of change of momentum = 10,020/0.4 = 25050 J = 25.05 kJ