When I apply a force on a body, it will apply a force on me (Newton's third law). But where does this force on me (i.e. equal but opposite force) generate from? Does it generate from within the body, or from somewhere else?

  • $\begingroup$ In the case of contact forces, microscopically speaking, it's because of the electromagnetic repulsion between electrical charges when the surface is compressed, but for distance molds such as gravitational ones, I'm also curious about why this reflexivity happens... $\endgroup$ Sep 8 at 1:20

It follows from the principle of conservation of momentum. Think of it as a transfer, like a payment. If you give me 20 dollars, I have 20 dollars more than I had before and you have 20 dollars less because you have transferred money to me. Giving me 20 dollars has an equal and opposite effect on the amount of money you have. It is the same with forces. If you apply a force to me, you transfer momentum to me- my momentum increases and yours decreases by an equal and opposite amount.

  • $\begingroup$ I like this, but how do you answer the objection that it works only if the objects are free to move, that is they are not acted upon by other (restraining) forces? $\endgroup$ Sep 7 at 20:51
  • $\begingroup$ If I push a pile of sand, I am applying force on it. The pile of sand will start to deteriorate, but will it be still applying the same force on my hand? Or the third law of motion assumes the objects involved are only rigid? $\endgroup$ Sep 7 at 20:59
  • $\begingroup$ Similarly, if I push air, why don't my hands experience equal but opposite force? Is it because the air is not rigid? $\endgroup$ Sep 7 at 21:12
  • $\begingroup$ @RameezUlHaq, who says it does not experience such forces? Calculate the force necessary to accelerate a few cubic centimeters of air from rest to the speed of your hand. That's a ballpark estimate of how much force you are exerting on the air. It is very difficult to feel that small of a force on your palm. It is low because air has little mass. Use a more massive fluid (like water) to generate stronger forces. $\endgroup$
    – BowlOfRed
    Sep 7 at 22:38
  • $\begingroup$ @BowlOfRed, so Newton's third law doesn't assume that the bodies in contact should be rigid? They can be flexible and locally deform when force is being applied by my hand onto the object and hence when my hand experiences an equal and opposite force? $\endgroup$ Sep 8 at 7:04

For example: I do a force on a broken car to move it. But instead of put my hand directly on it, I hold a spring, and there is another equal spring fixed on the car. So I do the force by contacting both springs.

No matter if the force is enough to move the car or not, the deflections of the springs are the same. So the forces are equal.

We could imagine a different outcome of the experiment without any logical contradiction I believe. So it is an experimental fact.


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