Answer to Question #86090 in Mechanics | Relativity for Rashwan Siddiqui

Question

Newton’s third law states that every action has equal and opptosite reaction. If this is true then how can we move any object? How can we stop any object? Logical answer according to the rules with one example please. Thanks

Answer 1

Yes, the forces between us and the object are equal, colinear and opposite. Consider the following.

These forces are applied to different bodies. Nothing moves when colinear opposite equal forces are applied to some point of one body. So when you write Newton's second law for one body (either a box or a man) these forces do not cancel each other, because one body has one "Third law" force, not two:

So why the body moves? Because the man exerts the muscle force F and it is greater than the body's force of friction f. If the force of friction f is smaller than the force F the pusher can apply, the body won't move. Draw a free body diagram for the purple fellow and you'll see the same thing about force from box's side and friction between the man and the green ground. Keep in mind that the force of friction depends on mass.

If you want to stop objects within a required distance, use Newton's second law for each body and again you'll see that everything depends on mass, friction and the distance. "Third law" forces do not cancel anywhere but applied to different bodies.