Question

if every action has a reaction then how does a body move??

Answer 1

Because actionf and reaction force are not exerting in the same body. If you push a box, the box exerts a reaction force, but upon you, not upon itself.

Answer 2

A body will move under the action of a force if it is free to move even if it is exerting a reaction force on the thing that is moving it. Lets look at several examples.

1. You do push ups on the floor. You exert a force on the floor and the floor on you. But you are free to move and thus you rise.

2. A spring is compressed by a clamp. both the spring and the clamp are not free to move and thus there is no motion. But you know that one is pushing on the other.

3. You push a cart and the cart pushes back put the cart is free to move so it does . You are pushing against the earth to prevent your motion.

4. An astronaut is working on a project in space and is carrying an item. He pushes the item and the item reacts in kind . Both are free to move so both the item and the astronaut will move ( in opposite directions).

5. You are holding an item. The item has a action force on you hand. But you exert the reaction which is equal and opposite to the items weight. there is no motion because you prevent your motion. If you raise the object you must produce an additional action (the item creates an additional reaction) and the item is free to move up. If the item moves it accelerates at least for a short time and the action (the force you are exerting) must be greater than the weight of the item (at least for a short time). Once it begins to move it will continue to move at a constant velocity if your force is now only equal to its weight.
Hold an object then lift it. You may notice ( you can use a spring scale to observe this) when you begin to lift it the reaction of the object increases at least momentarily as it begins to move but returns to the weight of the object when the velocity is constant.