Internal forces on a system of particles

1. Jan 12, 2010

johncena

I am very much confused with this statements
"The forces of action and reaction never cancels out each other , but the internal forces on a system of particles cancel out in pairs "
I think i know the reason for the first statement.i.e,The forces of action and reaction are acting on different bodies, and hence they does not cancel each other.
But what about internal forces ? Aren't they acting on different bodies ? then how can they cancel out each other ?

2. Jan 12, 2010

Staff: Mentor

Re: Confused !!!!

Let's say there are two bodies, A and B. A and B exert forces on each other which are equal and opposite. If you take your system to be A by itself, then the force that B exerts on A is an external force which contributes to the net force on A.

But if you take your system to be both objects together (A+B), then the forces that A and B exert on each other are internal forces. When you want the net force on the system A+B, those internal forces 'cancel out' since the force on A from B is exactly opposite to the force on B from A.

Make sense?

3. Jan 13, 2010

johncena

Re: Confused !!!!

I think i am still in confusion...
If you don't mind, can you demonstrate it by an animation ?

4. Jan 13, 2010

Staff: Mentor

Re: Confused !!!!

I don't know what you mean by 'an animation', but here's another attempt.

Lets say that B exerts a force +F on A, thus A exerts a force of -F on B. Let's assume that those forces are the only forces in the problem.

What's the net force on A? Answer: +F
What's the net force on B? Answer: -F

What's the net force on 'A+B' taken as a system? Answer: Just add the force on A (+F) and the force on B (-F) to get the total force on the system, thus +F -F = 0. The net force on 'A+B' is zero, since the only forces are internal.

These may help a bit: "[URL [Broken] Forces
[/URL] & "[URL [Broken] An Object and What Isn't?[/URL]

Last edited by a moderator: May 4, 2017
5. Jan 13, 2010

rcgldr

Re: Confused !!!!

The forces cancel only if you include the reaction forces to any internal accelerations due to deformations (compression or expansion) within an object.

6. Jan 15, 2010

yuqiao

Re: Confused !!!!

when you are refering to "internal forces", your subject is the system. thus, each internal force is exerted on the system. you can think of the system a mere point, and view the internal forces as the same as the external forces.
got it?

Last edited: Jan 16, 2010
7. Jan 18, 2010

johncena

Re: Confused !!!!

Yes!! I got it now ....Thanks.

Last edited by a moderator: May 4, 2017
8. Jan 26, 2010

Asok

Re: Confused !!!!

it is just a matter of what is your system...you have the total freedom to choose your system..if your system is a one particle , then you will have to take the internal forces only on it.. if the system is lot of particles then it is something like you are pushing your own hands against each other.. then you cant see any internal forces, they cancel out.. but as I first mentioned if it is a one hand , then you will have to take the 'force' by the other hand instead of taking the whole hand, because now your system is including only one hand...