Final answer:
The solid rubber ball sinks in water because it is denser than water, while the hollow rubber ball floats or stays on the water's surface due to its lower density.
Step-by-step explanation:
When a solid object, such as a rubber ball, sinks in water, it indicates that the object is denser than water. Density is defined as mass per unit volume. In this case, the solid rubber ball is denser than water, causing it to sink.
On the other hand, a hollow rubber ball with the same mass as the solid rubber ball may have a lower density. The empty space inside the hollow rubber ball reduces its overall density, allowing it to float or stay at the water's surface instead of sinking.
Therefore, the difference in density between the solid rubber ball and the hollow rubber ball is the reason why the solid ball sinks while the hollow ball floats or stays on the water's surface.