# Homework Help: Energy collision problem

1. Jun 30, 2008

### loststudent0

1. The problem statement, all variables and given/known data
A 100 g block on a frictionless table is firmly attached to one end of a spring with k = 20 N/m. The other end of the spring is anchored to the wall. A 20 g ball is thrown horiontally toward the block with a speed of 5.0 m/s.
a. If the collision is perfectly elastic, what is the ball's speed immediately after the collision?
b. What is the maximum compression of the spring?
c. Repeat parts a and b for the case of a perfectly inelastic collision.

2. Relevant equations
Us=.5k(delta s)2
K = .5mv2

3. The attempt at a solution
a. i'm thinking i have to use momentum of the ball.. but because of the spring, i guess i have to use energy equations..
Ki+Ui=Kf+Uf
but for some reason for the Ui, there is no place for me to put in the mass of the block since that is potential energy..do i need a mass?

i've found a thread thats the same..nm
https://www.physicsforums.com/showt...ed+direction+of+each+ball+after+the+collision

Last edited: Jun 30, 2008
2. Jun 30, 2008

### Staff: Mentor

To find the speed of the block after the collision, you'll need to apply both conservation of momentum and kinetic energy. For part (b) you'll need to consider spring potential energy.
Presumably the table is horizontal, so there are no changes in gravitational PE. The only PE you need to worry about is spring PE, which does not depend on mass.

3. Jun 30, 2008

### loststudent0

Thank you for you guidance Doc Al..i think i understand now