Why does it stop expanding when you stop blowing air into it

this is because of collisions of molecules present in that particular system.

Explanation:

For instance, when air is filled in a balloon then the number of the molecules of air inside the balloon increases. As a result more collisions of molecules take place with the walls. This gives rise to high air pressure on the walls which causes the balloon to expand and get inflated.

8 m/s because for him to get the same amount of energy as ray is putting out, bobby, who weighs half as much as ray, needs to run twice as fast.