= Body force
{wiki}
A <body force> acts throughout a body's or <fluid>'s <volume>, rather than only through its surface. <Gravity> is the example used in <hydrostatic pressure>: a <gravitational field> $\mathbf g$ gives <force> per unit <volume> $\rho\mathbf g$, where $\rho$ is the <mass density>. More generally, a <body force> per unit <mass> $\mathbf f$ contributes $\rho\mathbf f$ to <momentum conservation>. <Forces> from <electromagnetism> are another example. Surface <pressure> <forces> produce <buoyancy> on an immersed body, while its <body force> from <gravity> acts downward.
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