Click here to purchase the entire book in PDF format. Potential and Kinetic Energy Note: This section is actually just a necessary preface to the following sections. If you already know the difference between potential energy and kinetic energy, feel free to skip this explanation. Find a very lazy friend (preferably someone who doesn't weigh very much) and put him on a playground swing. He'll just sit there on the swing, which hangs by two ropes or chains, and he'll just keep sitting there until someone gives them a push (he's very lazy, remember?). This person has absolutely no energy. Then, you get behind him, and put your hands on his shoulders and start walking forwards, pushing him ahead of you. Then you stop. So, you're standing there, holding up your lazy friend who is sitting on a swing which is not yet swinging. You have put a lot of work into getting him all the way up in the air - in other words, you've expended a lot of energy while pushing him. However, that energy is not all lost... Rather than do all that work for nothing, what you've actually done is to transfer the work that you've done into your friend on the swing. He's now sitting there in mid-air, full of potential to start swinging. In other words, he has potential energy - the energy is there, ready to make him swing back and forth, but it's still only potentially useful because you haven't released him yet. So, once you get sick and tired of your friend complaining that he's not having any fun (then again, you weren't having any fun holding him up...) you let go and he starts moving back towards where he came from - downwards. As he gets closer and closer to the ground, he moves faster and faster. What is happening is that he's gaining kinetic energy - the energy of movement (kinetic means movement). The faster he moves, the more kinetic energy he has. What has happened in the process of him moving back to where he came from is that all of the potential energy you've put in him has been transferred into kinetic energy. When he's at the bottom of the arc, closest to the ground, he is moving the fastest, and cannot move any lower. Therefore all of the potential energy has been converted to kinetic energy. (He no longer has the potential to go any faster - so he's all out of potential energy.) However, he does have inertia - this is the tendency for a body in motion to remain in motion. So, he passes the bottom of the arc and keeps moving in the same direction, still going away from the place where you released him. However, as he moves upwards, away from the ground, he starts slowing down, thereby losing kinetic energy. But, since he's further away from the ground, he has the potential to come back, therefore his kinetic energy is being converted to potential energy. As he swings back and forth, he is constantly transferring potential energy into kinetic energy and back again. In a perfect world, he would keep swinging forever, trading potential for kinetic energy. However, in the real world we have friction which causes his kinetic energy to be converted into heat. The higher the friction, the faster this conversion happens. So eventually, even with a little friction in the system, all of the kinetic energy will be converted to heat and lost, and he'll stop swinging.