forcesnewtonian-mechanics
According to Newton's third law of motion, for every action there an equal and opposite reaction.
If an object A applies a force (action) on an object B, object B applies an equal and opposite force (reaction) on object A. If we consider this reaction force as the action force applied by object B on object A, shouldn't there be a reaction force for every reaction force and hence infinite reaction forces?
Imagine you are in a crowd of people. A huge crowd, with everyone, almost squeezed together. Imagine you run into the crowd. In that case, you will have an almost rigid body, because, you apply a force on him, he applies a force back on you, you stop, but he will have a force, which will be balanced by the next person and next and so on till you reach a wall.
But, if the crowd has a little less number of people, or more like people standing in rows holding hands to form a chain, it will be different. If you run into the crowd from outside, you will push a few people, and they will accelerate (inelastic collision) and you and the other person moves with the same velocity as you, but you are not falling into the person. He exerts an equal force, thus preventing you from sinking into him. But as there is no other person to provide an equal force on him, he starts falling in till he reaches another person because of the unbalanced forces on him
But that does not mean that he is not moving, because he is, and if you were large enough it would be similar to the case you talk about. Several people moving in a localized region that would 'appear' as if the forces are not equal. But if you go a bit more closer, you will see that it is not violated.
The sponge is not one body, rather it is like the crowd. The particles can move, to some extent, independent of the other particles. So, when you push a sponge, you are actually making some sponge mass to move, like crashing into the crowd.
So, Newton's third law is not violated
Best Answer
The problem is the misleading term reaction. This implies that force A (the action) exists before its paired force B (the reaction), and causes force B to come into existence in response to applied force A.
This is wrong. Forces are always created in pairs, acting on different bodies. These paired forces describe the separate halves of an interaction between the two bodies.
When object A collides with object B, it transfers momentum to B but it does not transfer force. A has momentum before it comes into contact with B, but the force which A exerts on B does not exist until the balls come into contact. The paired forces always come into existence at the same time and cease to exist at the same time. At every instant they are always equal and opposite; they are simultaneously the cause and effect of each other.