# Relative velocities

1. Jun 27, 2010

### mandy9008

1. The problem statement, all variables and given/known data
A 45.0 kg girl is standing on a 163 kg plank. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.52 m/s relative to the plank.
(a) What is her velocity relative to the surface of ice?
(b) What is the velocity of the plank relative to the surface of ice?

2. Relevant equations
m1v1i + m2v2i = m1v1f + m2v2f

3. The attempt at a solution
a. (45.0 kg)(0 m/s) + (163 kg)(0 m/s) = (45.0 kg)(v1f) + (163 kg)(1.52 m/s)
v1f=5.51 m/s

2. Jun 27, 2010

### kuruman

Good beginning, but you need to express the velocity of the girl relative to the ground, not relative to the plank when you conserve momentum.

3. Jun 27, 2010

### mandy9008

so how would I go about doing that?

4. Jun 28, 2010

### kuruman

Say the girl is moving left to right. In what direction is the plank moving?