# If Newton’s 3rd law is true, how is there movement?

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Newton’s 3rd law states something along the lines of “every action has an equal and opposite reaction” but if that’s true then how is there movement? wouldn’t it get cancelled out by the opposite reaction?

In: Physics

In short, because the earth is massive. If I were to jump on a table, my legs push the table downwards, the table ‘pushes’ my body upwards and the earth downwards, but the earth is MUCH bigger than me so the effect I have is negligible. If you were in space or on a perfectly frictionless sheet of ice, any attempted movement with cause you to thrash roughly in place.

The opposite reaction to movement is just movement in the opposite direction. The law doesn’t really forbid movement itself, but says that you get two movements for the price of one (ok not always cuz it deals with force and a lot of time you can apply force to an object without moving it)

The “opposite reaction” acts on a different “body” (object) – if you and a friend are standing on ice, and you push your friend forwards, there will be a reaction force which pushes you backwards, away from him.

Was answered pretty well above, but I have an example I use in class all the time.

When I walk, I push backwards in the concrete (action) – which does not cause the concrete go accelerate notocibly due to its mass and other forces acting on the concrete.

The concrete must then push forward on me with an equal amount of force (reaction) – which causes me to accelerate forward.

Equal and opposite forces are acting on opposite things, and the equal forces does not lead to an equal result in the force.

Newton’s third law does not deal with movement by itself, but with the change in movement, which is acceleration or deceleration. In fact, Newton’s first law states that “an object in rest will stay at rest and an object in a constant motion will keep its constant motion unless it is acted upon by an external force”. So if you are playing marbles, you need some force to shoot your marble, but once the marble is rolling, it will continue rolling by itself. (Provided of course an ideal environment where there is no friction from the ground and no resistance from the air). The second law states that the faster the acceleration you want the more force you need, and also the heavier the object you want to accelerate, the more force you need. And this is linear: double the acceleration needs double the force, double the mass needs double thr force. In fact, taken to the point, the second law states that force needed is acceleration times mass.

The third law finally states that in a closed system force can not come from nothing. If one object applies a force to another, that second will extend tr same force back onto the first. So if you put your hand in front of a rolling marble, you will stop the marble, but you will feel the force of the marble on your hand. If you put another marble in front of the rolling marble, it will deflect the marble, but as a result yue other marble will be forced into moving itself.

So the third law does not make movement impossible. It simply states that the force needed for movement to start or stop or change will extert a similar force to the object applying it. Movement does not need force and change in movement: acceleration, deceleration or change in direction just result in an opposite force applied to the object, system or person forcing the change.