Conservation of momentum of a frog jump

1. Mar 29, 2015

Luca 123

1. The problem statement, all variables and given/known data
A frog of mass m jumps from the edge of a plank of mass M of length L to the other edge. The plank is on water and is free to move, assume no friction whatsoever. What is the minimum value of the speed of the frog ?

2. Relevant equations
Conservation of momentum and energy
Some kinematics equations

3. The attempt at a solution
I think that the minimum speed means the magnitude of the velocity at the highest point, meaning Vx. But I cannot find

2. Mar 29, 2015

Staff: Mentor

What have you tried?

Have you determined how far the frog must jump?

3. Mar 29, 2015

Fix the end of plank without frog as (0,0) and let the frog be at (-L,0).
What will happen when the frog jumps?
What is center of mass of the system?
Using the center of mass, calculate the distance the frog has to jump.

4. Mar 29, 2015

haruspex

No, it means the minimum take-off speed. Remember that the frog can choose what angle to take off at.