// physics › Newton's Laws
See Newton's third law: every action force has an equal and opposite reaction on a different body.
F(A on B) = − F(B on A)
A mind behind this: Isaac Newton 1642–1727
Every push comes with a push back. Whenever you push on something, it pushes back on you just as hard, in the opposite direction. People say it as: 'for every action there is an equal and opposite reaction'.
This is the trick that confuses everyone! The two forces push on DIFFERENT objects, so they cannot cancel. When you push a wall, one force is on the wall and the other is on you. They only cancel if they act on the same object — and these do not.
A swimmer pushes the water backward with their hands; the water pushes the swimmer forward. That forward push from the water is what moves them through the pool. The swimmer cannot push themselves — they push the water, and the water pushes back.
A rocket pushes hot gas downward out of its engine. By the third law, the gas pushes the rocket upward with an equal force. That upward reaction is what lifts the rocket — it does not need anything to 'push against', which is why rockets work even in empty space.
Same law. The gun pushes the bullet forward; the bullet pushes the gun backward with an equal force. That backward push is the 'recoil' or 'kick' you feel in your shoulder.
Yes, always exactly equal in size and exactly opposite in direction — no matter how big or small, heavy or light the two objects are. A mosquito hitting a truck pushes the truck just as hard as the truck pushes the mosquito (the mosquito just feels it a lot more!).