The answer to your question depends largely on what example you're talking about, so letís talk about the example you gave.
When you throw a ball, a lot more things happen than meet the eye. First of all, the ball gets its energy of motion from your hand which pushes it. The muscles in your hand and arm get their energy from a chemical reaction involving a compound called ATP. ATP is the way that your body stores the energy it gets from the food you eat.
Ok, so now the ball is flying through the air - why does it keep on moving? Well, the ball has a certain amount of energy. This is what determines how high up it is and how fast itís going. The higher up it is, the more potential
energy it has, and the faster itís going the more kinetic
energy it has. Energy can go back and forth between these two, but the total wonít change (unless the energy is transfered to something else). So the ball can be really high up and moving slowly or close to the ground and moving faster.
Now youíre probably going "Sure, so the ball will keep moving - thatís inertia. But we all know it wonít go forever - what stops it?" Well, it stops when it runs into something - when that happens, the energy that had been making it move is transfered into the thing it hit. The most obvious thing for it to hit is something big - the ground, another personís hand, etc. Less obvious is the air itself! As the ball moves, it runs into a LOT of little air molecules. Sure, theyíre really little so they donít slow it down much, but it does make a difference. (This is especially convenient if you plan on taking up Skydiving
Hope this helps!
(republished on 07/13/06)