# Homework Help: Ice surfing

1. Oct 28, 2004

### Chileboy

Ice surfing!!!

Hi, this is the problem:

A small boy places a plank of wood on a frozen pond and then, taking a run up, jumps onto one end of the plank. If the plank is perpendicular to the boys trajectory and there is no friction between the plank and the ice, How will the plank move after the boy has landed on it?

This is what I was thinking of doing:
-Calculate the center of mass
-Calculate the moment of inertia (rotating around the center of mass)
-Calculate the speed using conservation of momentum ( mVo = (m+M)V ).
-Equal the boys initial kinetic energy to the final kinetic energy (using V calculated earlier) plus the rotational energy and then finding the systems angular speed.

:grumpy: The problem is that this obviously flawed thinking doesn't take into account the fact that the plank is at a certain angle initially, and intuitively I can see that if the plank were parallel to the boys trajectory it wouldn't rotate at all, so i am obviously missing something.

I'd appreciate any help

2. Oct 28, 2004

### Galileo

If the trajectory is perpendicular to the plank, wouldn't the velocity of the boy be perpendicular to the plank (and the surface of the ice)? Which means the force on the plank will be perpendicular to the surface and so the plank won't move at all. (Except through the ice maybe).

3. Oct 28, 2004

### Hurkyl

Staff Emeritus
Galileo: Think in three dimensions!

Last edited: Oct 28, 2004
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