Question

A 1200 kg truck is rear-ended by an 800 kg car. Before the collision, the car is traveling at 30 m/s and the truck is going
20 m/s. After the collision the car is traveling 23 m/s. If momentum is conserved, how fast is the truck traveling?
Use units in ALL calculations. 24.67 m/s

First, label the quantities as shown in the diagram:

m1 = _________ m2 = _________
u1 = _________ u2 = _________
v1 = _________ v2 = _________

Answer 1

m1 = 800 kg (car)
m2 = 1200 kg (truck)
u1 = 30 m/s (car initial)
u2 = 20 m/s (truck initial)
v1 = 23 m/s (car final)
v2 = 24.67 m/s (truck final)

Check (momentum conserved):
m1 u1 + m2 u2 = 800·30 + 1200·20 = 24000 kg·m/s + 24000 kg·m/s = 48000 kg·m/s.
m1 v1 + m2 v2 = 800·23 + 1200·24.67 = 18400 kg·m/s + 29600 kg·m/s = 48000 kg·m/s.