Chan Hee is inflating a basketball using an air pump. He notices that the pump gets warm as he uses it. What is a good hypothesis that could lead to new experimentation?

If there is no air in a basketball, then it is hard to play the game.
A basketball that is inflated bounces better than one that is not inflated.
If air in a pump is squeezed more, then the air gets hotter because energy is added to it.
Basketball is more fun to play than baseball because you have to move a lot more.

If air in a pump is squeezed more, then the air gets hotter because energy is added to it.

Explanation:

A hypothesis is a simple scientific guess that can be investigated. It usually adopts the if -then and because approach to relate variables to the expect output.

The most fitting statement of hypothesis that can be investigated is that "If air in a pump is squeezed more, then the air gets hotter because energy is added to it".

We can find the volume of the air, the pressure of the pump and temperature and discern their relationships.

Based on the kinetic theory, pressure causes increasing collisions between particles and hereby, a gas will be hotter.