Every action has an equal or opposite reaction. The meaning of this is that forces always act in pairs. First, an action occurs, then a reaction occurs that has the same amount of strength to the action, but it happens in the opposite direction. If there are two people on a skateboard and they push eachother, they move apart. The harder they push, the farther they go.
Momentum
This is a property of some kind of moving object. It makes the object harder to stop. Momentum equals mass times velocity. Velocity is how fast something goes. An object that has greater mass, has greater momentum, velocity, or both.
