# Homework Help: Kinetic & Potential Energy / Momentum Problem

1. Nov 29, 2011

### x93

1. The problem statement, all variables and given/known data
From Kleppner and Kolenkow:
A small cube of mass m slides down a circular path of radius R cut into a large block of mass M, as shown. M rests on a table, and both blocks move without friction. The blocks are initially at rest, and m starts from the top of the path.
Find the velocity v of the cube as it leaves the block.

Ans. clue. If m=M, v=√(gR)

http://img390.imageshack.us/img390/5575/blocksbi2.jpg [Broken]

2. Relevant equations
KE=(1/2)mv2
PE=mgh
mivi=mfvf
KEi=KEf

3. The attempt at a solution
For m, initial energy PEm=mgR
Just before m leaves block M, KEm=(1/2)mv2=mgR
Thus, v=√(2gR).
From here, I set up equations for conservation of momentum as well as kinetic energy.
I then used substitution and tried to solve for the velocity of m as it leaves the block, M, but the algebra just isn't working out. Did I make a mistake?

Last edited by a moderator: May 5, 2017
2. Nov 30, 2011

### ehild

Do not ignore the KE of the large block.

ehild