Figure 9-1 Hydrostatic Equilibrium (a) Material in the Sun’s interior tends to move neither up nor down. The upward forces on an imaginary slab of solar material (due to forces from gases pushing up below the slab) must balance the downward forces (due to the slab’s weight and the weight of gases above the slab). Hence, the pressure must increase with increasing depth. (b) The same principle applies to a fish floating in water. In equilibrium, the forces balance and the fish neither rises nor sinks.