Conservation of momentum of a frog jump

  • #1
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Homework Statement


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 ?

Homework Equations


Conservation of momentum and energy
Some kinematics equations
The answer given is sqrt((MgL)/(m+M))

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 [/B]
 

Answers and Replies

  • #2
gneill
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What have you tried?

Have you determined how far the frog must jump?
 
  • #3
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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
haruspex
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I think that the minimum speed means the magnitude of the velocity at the highest point
No, it means the minimum take-off speed. Remember that the frog can choose what angle to take off at.
 

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