# Prove the average external force is Zero

1.Problem statement
Prove the average external force of a system of particles N starting from rest and ending at rest is zero.

## Homework Equations

If the system moves periodically prove the av. external force is zero in a period

## The Attempt at a Solution

I dont quite understand what i am asked .Since i dont have a definition for a average force of a system of particles. Yet i tried to derive the eq for momentum of a system.So this is the momentum of the system i found. Now i know that going from rest to rest change of momentum must be zero . But i have internal and external forces. So dont know how to proceed #### Attachments

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BvU
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There is someething with internal forces you need to exploit here. has to do with one of the Newton laws...:
what is the effect of an internal force on the motion of the system ?

There is someething with internal forces you need to exploit here. has to do with one of the Newton laws...:
what is the effect of an internal force on the motion of the system ?
Its zero the rest of the math is ok? SO the double sum is zero.

BvU