## Momentum

The*momentum*of a mass is usually defined by

*vectors*in 3D space. We will return to vector momentum in §B.4.1 below.

### Conservation of Momentum

Like energy, momentum is*conserved*in physical systems. For example, when two masses collide and recoil from each other, the total momentum after the collision equals that before the collision. Since the momentum is a three-dimensional vector in Euclidean space, momentum conservation provides three simultaneous equations, in general.

^{B.10}

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Rigid-Body Dynamics

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Work = Force times Distance = Energy