Why does action and reaction really works?


When you throw an object away in space, why do you accelerate as well instead of just transferring all energy to the movement of the object?

In: Physics

Because that object has mass as well.

Simple example – if an object has equal mass to you and you press against it, the force applied is going to be split 50/50 between pushing the object away from you and pushing yourself away from the object.

So even a tiny object (like a ball) is going to have some mass to it, which means when you throw it, it may receive 99% of the force (because its mass is so small) which leaves 1% to be applied to you in the opposite direction, but that’s still 1% more than you had before you threw the object.

There isn’t precisely a “why” in the sense that there’s a reason behind it; we don’t know *why* momentum is conserved, it just is.

In this case, momentum being conserved means that, for every bit of momentum given to the object, an equal and opposing amount of momentum must also be transferred to you, and as a result you end up moving in the opposite direction.

Because when you push the object, the object is pushing you. From *your* standpoint, you’re sitting in space and the object is pushing you, so you move away. That takes some of the energy, so it can’t all go to the object.

When you jump, you’re pushing off the earth. You get pushed up, the earth gets pushed down. You move a lot, because you’re light (relative to earth). The earth moves an incredibly tiny amount (but it’s not zero) in the opposite direction so that momentum balances. The mass imbalance is so huge that virtually all the energy goes to you, not the earth, so much so that we almost always ignore the earth side in an energy balance, but it can still never be 100% in only one of the two objects because they’re *both* getting pushed and momentum has to be conserved.