Two masses of 2.0 kg and 42.0 kg, resting on a frictionless table, are connected by a compressed spring. When the spring is released, the smaller mass has a velocity of 13.0 m/s.

Question

What is the velocity of the larger mass?

Answer 1

Initially, the total momentum of the 2 masses was zero. The spring force between them is an internal force, so momentum is conserved when the masses are released. Therefore, the momentum of one is equal in magnitude but opposite in direction to the other.

m₂v₂ = -m₁v₁

Now plug in

m₁ = 2.0 kg

m₂ = 42.0 kg

v₁ = 13.0 m/s

and get v₂ in m/s. Keep in mind the direction of v₂ is opposite the direction of v₁.