# Conservation of momentum help

## Homework Statement

so, i have a problem. see the pic
http://img255.imageshack.us/img255/4628/aughhm9.png [Broken]
the question is. if we have, for example 10 balls standin like that, if we move the ball which is on the right, only one will move, the left one. if we move two of them from the right, two of them will move on the left also.

## Homework Equations

Ft = mv to satisfy the conservation of momentum

## The Attempt at a Solution

the solution is that something else is satisfied, i know that the momentum is satisfied but something is missing. please help :-)

thanks in forward

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## Answers and Replies

Doc Al
Mentor
Mechanical energy is also conserved.

I dont get your question. What exactly are you asking?

From Wikipedia
http://en.wikipedia.org/wiki/Newton's_cradle

The behaviour of the pendulum follows from the conservation of momentum and energy only in the case of two pendula. Indeed, if there are r pendula there are also r unknown velocities to be calculated from the initial conditions. An additional condition for the observed outcome is that a shock wave has to propagate dispersionfree through the chain.

Doc Al
Mentor
It is certainly true that it takes more than simple conservation of momentum and energy to derive the observed behavior of Newton's cradle. Most textbooks pull a fast one and just assume that however many balls move after the collision, they all move together. (And no rebounding.)

Using this you can show that the number of balls swung on the left will equal the number of balls that will emerge on the right.