Prove the average external force is Zero

  • #1
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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
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Answers and Replies

  • #2
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 ?
 
  • #3
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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 ?
Its zero the rest of the math is ok? SO the double sum is zero.
 
  • #4
BvU
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Can't read the small writing. What I hinted at is action = - reaction so the internal forces can't influence the center of mass of the system
 

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