Answer:
Gravitational potential energy is greater for the rock [tex]20\; {\rm m}[/tex] above the river.
Explanation:
The gravitational field near the surface of the earth is approximately uniform. Let [tex]g[/tex] denote the strength of the gravitational field near the surface of the earth.
Let [tex]m[/tex] denote the mass of each rock. Assume that the gravitational potential energy of the two rocks is [tex]0[/tex] when the rocks are at the surface of the river.
When the rock is at a height of [tex]h[/tex] above the river, the gravitational potential energy ([tex]\text{GPE}[/tex]) of that rock would be [tex]\text{GPE} = m\, g\, h[/tex]. In other words, the [tex]\text{GPE}\![/tex] of the rock is proportional to its height relative to the ground.
Hence, among the two rocks, [tex]\text{GPE}[/tex] would be greater for the rock that is higher above the ground ([tex]20\; {\rm m}[/tex]) than the rock [tex]10\; {\rm m}[/tex] above the ground.
