If you've exercised before, you can probably remember the feeling in your body when you're finished--the 'afterglow' of muscle fatigue, endorphins, and heightened metabolism--and you can visualize that. If you haven't, or can't remember, you can imagine feelings in your mind like confidence and self-satisfaction that you'll have at the end of the exercise.

As for studying, the goal isn't to study, per se; it's to do well on the test. Visualizing the emotional rewards of success on the test itself can motivate you to study, as well as get enough sleep the night before, eat appropriately the day of, take performance enhancing drugs, etc.

Imagination is a funny thing. You can imagine things that could physically never happen--but if you try to imagine something that's emotionally implausible to you, you'll likely fail. Just now I imagined moving objects with my mind, with no trouble at all; then I tried to imagine smacking my mother in the face and failed utterly. If you actually try to imagine having something--not just think about trying--and fail, it's probably because deep down you don't believe you could ever have it.