## 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