Question

A 225 kg red bumper car is moving at 3.0 m/s. It hits a stationary 180 kg blue bumper car. The red and blue bumper cars combine and hit a stationary green bumper car of mass 150 kg. The red, blue, and green bumper cars all combine.

What is the final velocity of the combined bumper cars?

0.93 m/s
1.2 m/s
1.7 m/s
2.7 m/s

Answer 1

Use conservation of momentum for each perfectly inelastic collision.

After red (225 kg) hits stationary blue (180 kg):
v1 = (225·3.0 + 180·0) / (225+180) = 675 / 405 = 1.6667 m/s ≈ 1.7 m/s.

Then red+blue (405 kg) hit stationary green (150 kg):
vfinal = (405·1.6667 + 150·0) / (405+150) = 675 / 555 = 1.2162 m/s ≈ 1.2 m/s.

Answer: 1.2 m/s.