Momentum is a quantity that expresses the motion of a body, equal to the product of its mass and velocity. The momentum of an object depends on its mass and velocity, with greater mass or velocity resulting in more momentum. The law of conservation of momentum states that in a closed system without external forces, the total momentum before and after an interaction will be the same. Examples include a person recoiling after firing a gun or moving backward when throwing an object off a skateboard. In collisions, the total momentum of the system is conserved and can be expressed mathematically as the sum of the momentum of the objects before equalling the sum after.