# An icy situation

1. Oct 5, 2005

### willworkforfood

A 47.7 kg person is standing on a 77 kg plank. The plank is originally at rest on a frozen, frictionless lake. The person begins to walk along the plank at a constant velocity of 2.06 m/s to the right relative to the plank.

What is the plank's velocity relative to the ice surface in m/s?

I realize this is a momentum problem, I just don't know how to solve for the velocity of the plank with respect to the ice or of the person with respect to the ice or what a "right relative to the plank" is...

Any help is appreciated as always

Last edited: Oct 5, 2005
2. Oct 5, 2005

### Päällikkö

Draw a diagram. If the planck's velocity is v2 (relative to ice) into the opposite direction, what is the person's velocity (relative to ice)?

3. Oct 5, 2005

### StatusX

One way to do this problem is to consider it in the moving board frame. That is, if the board is moving with a speed v to the left (with respect to the ice, since the person is moving to the right), then in the frame with speed v to the left you know the initial and final momenta (ie, before the person starts walking, the person and board move right (in the frame) at a speed v, and afterwards just the person is moving at the given speed) and you know momentum is conserved, so you can solve for v.