Two cars are involved in a rear-end collision. A Hummer (m = 2500 kg) is traveling at 20 m/s when it hits a Smart Car (m = 1000 kg) that is stopped at a red light.

Question

Two cars are involved in a rear-end collision. A Hummer (m = 2500 kg) is traveling at 20 m/s when it hits a Smart Car (m = 1000 kg) that is stopped at a red light. If the Hummer comes to rest, how quickly is the Smart Car moving after the collision?

Answer 1

Momentum is conserved meaning the total momentum before the collision is equal to the total momentum after the collision as long as there are no external forces involved.

Let "v" be the velocity of the Smart Car after the collision.

Momentum before the collision is the mass of the Hummer times the velocity of the Hummer PLUS the mass of the Smart Car times the velocity of the Smart Car or (2500)(20) + (1000)(0) = 50,000 kgm/s

Momentum after the collision is (2500)(0) + 1000v.= 1000v

So 1000v = 50,000

v = 50000/1000 = 50 m/s