Well I am still small student of physics, but as far as I know, Newton's Third Law states that,

"For every action there is equal and opposite reaction."

Now let me tell you how I understood this,

Suppose there are two bodies A & B initially at rest (w.r.t the earth), now if A makes collision with B, of course there will be some force acting on A->B (say $F_{ab}$) and according to the third law there would be some reacting force from B->A (say $F_{ba}$). And both would be numerically equal. But now just consider body A, it has some acting force on it i.e. $F_{ba}$ which is being given by body B, so won't reaction produce for that force? Is that reaction is nothing but $F_{ab}$?

Now for both bodies, there should be some static frictional force (before movement) acting against the force which (tends to) move the body, so whenever two bodies act with each other, are there 4 force produced?

  • 1
    $\begingroup$ Could you please provide a drawing/diagram of what you're considering? Your description isn't completely clear (to me). $\endgroup$
    – Danu
    Commented Mar 3, 2014 at 18:09
  • $\begingroup$ i'll try about drawing but here is more clear detail.."A collides with B - reaction is produced from b to a, question is, would there be reaction for that reacting force" $\endgroup$
    – Deep
    Commented Mar 3, 2014 at 18:09
  • $\begingroup$ You could say, as you suggest, that reaction force of B on A is $F_{ab}$. But I think you are still fuzzy on the concept. @WillO has said it best. The statement of the third law that you quote at the top is opaque and possibly meaningless, since we have to guess what "action" and "reaction" mean. There are many better statements that might help; the Wikipedia entry for Newtown's Laws has a very good one that I recommend to you. Now... friction: what friction are you talking about? I don't see any in your problem. $\endgroup$
    – garyp
    Commented Mar 3, 2014 at 18:40
  • $\begingroup$ If you like this question you may also enjoy reading this post. $\endgroup$
    – Qmechanic
    Commented Mar 3, 2014 at 21:37

3 Answers 3


Your mistake is to believe that every action causes an equal and opposite reaction, in which case the reaction would cause a re-reaction, etc.

Instead, the right way to think about Newton's Third Law is that whatever causes an action must simultaneously cause an equal and opposite reaction. The action/reaction don't cause each other; they are both caused by some force, which, having caused them, is under no obligation to cause anything else.


"For every action, there is an equal and opposite reaction"

This means, forces exist as pairs.

When there is an interaction between $A$ and $B$, an action-reaction pair between them is produced. Which one is action and which one is reaction depends on your frame of reference.

Now to the static friction. Your question is pretty vague so I'm going to cover two cases

Case (A): Both bodies rest on a floor, along which they are pushed.

In this case, the friction of each body is between the body and the floor. So you will have three action-reaction pairs: Between A and B, between A and the floor and between B and the floor.

Case (B): A rubs against B when it moves.

In this case, there is a frictional force between A and B. This, too, occurs as an action-reaction pair.

Hope this makes sense to you.


This is the answer for one of previous question,

It should be "Every action has equal and opposite reaction of SAME TYPE" , the best way to understand these contact force related questions is to draw a large clear free body diagram. Which will eventually lights up your problems.

Draw all possible forces on both object(s) and or contact surface.

Pls see the image , the force you applied is not belong to the action, it is a separate force, action reaction forces are (Ra and Rb) their magnitude remains equal regardless of situation (acceleration, declaration, constant velocity etc) , and those are the action reaction pair, not the force you applied on the body.

If you are concerning about the reaction force due to 5N force applied on the object A , it will be acting on your hand which is still 5N acting opposite direction. Also to clarify your concern, newton pair of force always acts on different bodies not on the same bodyenter image description here


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