# Horizontal elastic collision between a ball and a block attached to a spring?

## Homework Statement

The problem states: A 0.140 kg block on a frictionless table is firmly attached to one end of a spring with k = 25 N/m. The other end of the spring is anchored to the wall. A 0.023 kg ball is thrown horizontally toward the block with a speed of 5.2 m/s. What is the ball's speed immediately after the collision (if the collision is perfectly elastic)?

## Homework Equations

F(spring) = -k(change in s)
K(f) + U(f) = K(i) + U(i) ...---> K = (1/2)mv^2 ...U = (1/2)k(change s)^2

## The Attempt at a Solution

The one thing that prevents me from solving the problem is that I am unable to find the length of the compressed spring, and I also do not know how to use F = -k(change s) in this problem. I know that since this is perfectly ellastic, I should use conservation of momentum or conservation of energy, but I always come out with two unknown variables.... the final velocity of the ball and usually, change in s. Any pointers? Thanks so much.