When you push a wall, the wall pushes you back. This is Newton's third law. It explains how a swimmer moves forward and why a gun moves backward when it fires. At the end we see why the total momentum of colliding objects does not change.
3 lessons, about 1 h 5 min3 simulations12 problems6 practice questionsPython in every lesson
Forces always come in pairs. When one object pushes another, the second object pushes back with a force of the same size in the opposite direction.
When no outside force acts on a group of objects, their total momentum does not change. We use this for collisions and for recoil.
A rocket moves in empty space because it throws gas backward. The gas pushes the rocket forward. This is the third law and conservation of momentum together.
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