If every action has an equal and opposite reaction then


Why does anything move at a local and universe scale ? Shouldn’t we have achieved equilibrium and perpetual rest ?

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The “opposite” means “opposite direction.”

Basically, when you shove a wall, the reason why you fall backwards rather than staying upright is because you can’t shove the wall without also shoving yourself away from the wall.

Yeah, balance means equilibrium, but it does not mean static equilibrium for every object. The classic example of momentum in a classroom: you are standing on a frictionless frozen lake holding a backpack. You can’t run or walk because there’s no friction on the ice. To get to the edge of the lake, you throw your backpack. If you weigh 10 times what your backpack does, you will fly off at 1:10 for speed in the opposite direction. Neither you or the backpack are stationary, but the entire system of you in the backpack is balanced.

But in the real world, you will come to a stop, because there is air resistance and there will be some friction on the ice. But now the air is moving a little bit more, the planet is spinning a little bit differently, and the ice underneath your feet is a little bit warmer. All still in equilibrium, but on a wider scale.

But there is a lot of energy in the universe, so it takes a long time for it to spread out evenly. We are riding a swinging pendulum that hasn’t settled yet.

Eventually, yes. This is the heat death of the universe.

Until then while systems have uneven energy, they are able to exert forces on each other.

You push on a wall.

The wall pushes on you with the same amount of force.

If you are not anchored, you will push away from the wall because the wall has more forces keeping it “stationary” than you do, so you essentially have all of the energy you push into the wall pished back at you, so you fall over.

There are tons of air molecules and all that behind you. You exert force on them and they exert force on you. They are less anchored than you are, and so they are forced to move out if the way. The accumulation of forces that these air mopecules cause is why we have “terminal velocity.” That is the point where the force exerted by a falling body is equaled by the force exerted by the air it is falling into, causing the accelleration to cease and a constant speed to be maintained.

But you are also technically correct. Eventually, it is theorized, every energy system will equalize. There is so much energy in some things that that will take billions of years still.

Until then, you have to remember that this “equal and opposite reaction” happens to every particle of a body, so many more forces are at play than just pushing on the wall. It also doesn’t account for density and friction and other forces.