Question

A skateboard is sitting at rest in the street. A 45 kg sixth grader gets a running start at 4.3 m/s and jumps on the 8 kg skateboard, and they both roll down the street. How fast will they be moving together down the street?

193.5 m/s

24 m/s

0.27 m/s

3.6 m/s

Answer 1

Use conservation of momentum (the jump is an inelastic collision):

v = (m1 v1 + m2 v2)/(m1 + m2) = (45·4.3 + 8·0)/(45 + 8) = 193.5/53 ≈ 3.65 m/s.

Answer: 3.6 m/s.