What are the speeds and compression in a collision with a spring?

[loststudent0]

Homework Statement

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.

Homework Equations

U_s=.5k(delta s)²
K = .5mv²

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..
K_i+U_i=K_f+U_f
but for some reason for the U_i, 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 that's the same..nm
https://www.physicsforums.com/showt...ed+direction+of+each+ball+after+the+collision

 

[Doc Al]

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..
K_i+U_i=K_f+U_f

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.

but for some reason for the U_i, there is no place for me to put in the mass of the block since that is potential energy..do i need a mass?

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.

 

[loststudent0]

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