# Trouble with Newton's Third Law of Motion

1. Jan 29, 2015

### Daniel Wqw

Please forgive me for my naiveté but I've got to resolve a conceptual problem I'm having.

If every action has an equal and opposite reaction then why don't all the forces in the universe cancel each other out.

For example if i am in space and I hit a ball with force X then the ball reacts back with force X which would mean the forces cancel each other meaning there would be no resultant force and therefore no movement. I know this is nonsense but I'm just giving you an idea of the conceptual difficulty I'm having.

If anyone could resolve this I would be very appreciative. Thanks!

2. Jan 29, 2015

### Satvik Pandey

Hi Daniel.

Action and reaction forces acts on two different bodies so they never cancel out.

3. Jan 29, 2015

### Bandersnatch

The two forces are not applied to the same body. When kicking a ball you act on the ball with force F and the ball acts on your foot with force -F.

4. Jan 29, 2015

### siddharth23

Consider this scenario. You slap your friend. Your friend slaps you back with the same force. Equal and opposite. If they were to cancel each other out, you wouldn't feel a thing. Try it

5. Jan 29, 2015

### A.T.

Yeah, the right way to cancel forces is to have two friends slap you simultaneously from opposite sides. Report back if you felt anything.

6. Jan 29, 2015

### Daniel Wqw

I'm sorry but I still really don't understand. Could somebody give a physical example please?

7. Jan 29, 2015

### Bandersnatch

A block of mass M1 is sliding on a frictionless surface with constant velocity V. It collides with a stationary block of mass M2.
During the collision (duration t), block M1 exerts force F1 on block M2. At the same time, block M2 exerts force F2 on block M1. $$\vec{F_1}=-\vec{F_2}$$
As a result of the two forces, each acting on a different block during the collision, block M1 will have lost momentum and block M2 will have gained momentum. Total momentum is conserved.

8. Jan 29, 2015

### A.T.

Search the forum or the www for "horse cart paradox".

9. Jan 29, 2015

### Daniel Wqw

YES! I got it. You only resolve one objects forces, not two objects. If you resolve two objects forces the forces do cancel but if you resolve each objects forces individually then you get the right result.
Ok my fault coming from an incorrect assumption you can resolve two objects forces together when the whole principle of resolving forces rests on the fact that they're all acting on one object! At least I think this explains my error.
I actually realised this from thinking about the horse cart paradox which I found.
What everyone says actually makes sense now and I didn't see the implications of the fact the opposite forces are acting on different objects!

Thanks everyone for your help except for siddharth23's advice which, if I didn't know better, would create more problems than I'm trying to solve :)

10. Jan 29, 2015

### siddharth23

Haha. I wouldn't expect you to do it. Just imagine it. Anyways, as long as you got it..

11. Jan 31, 2015

### Andrew Mason

If you consider the force on the two bodies together by looking at the acceleration of the centre of mass of the two interacting bodies, the forces do indeed result in a net zero force (on the two bodies as a whole). That leads to the principle that momentum is always conserved in any interaction. If you take the universe as a whole (which is a bit hard to define given present knowledge) the net force on the universe due to all interactions would be zero. So in that sense all the forces in the universe should sum to zero and, in that sense, cancel each other out.

AM

12. Feb 2, 2015

### CWatters

The forces don't cancel because they act on different objects. One acts on the ball and the other acts on you. You go in the opposite direction to the ball.

13. Feb 2, 2015

### psuedoben

the way i see it is, if there wasn't an equal but opposite reaction, the ball would simply pass through the object you used to hit it with. the equal but opposite reaction means that the ball won't move relative to the object used to hit it with during the contact of the two. hope this helps a little