Mechanical energy

This looks like a swing set. Yes, it is. But not just that. It’s also a physics experiment. When the swing is in this position, it’s charged with energy.

Since it’s positioned so high up, it can move downward, without us having to add any energy at all. We have a possibility, a potential, to release this energy. So we can call it potential energy. We let go of the swing, and release the potential energy. Pause here!

Now, with the swing in its lowest position, it doesn’t have any potential energy at all. But it’s in motion, so it has another form of energy: the energy of motion. Or, using a proper term, Kinetic energy. So, roll the film again. Stop right here!

The kinetic energy pushed the swing forward and upward, against the force of gravity. What kind of energy does the swing have now? Potential energy! The energy in the swing is switching between potential and kinetic, back and forth. At its high point, it has only potential energy.

And at its low point, it has only kinetic energy. Look at this diagram. When the potential energy is at its highest, the kinetic energy is at its lowest. And vice versa. The total energy of the swing -- that’s the one you get if you add the potential and kinetic energy -- is constant.

It’s conserved. And this is called mechanical energy. Mechanical energy equals potential energy plus kinetic energy. And the mechanical energy is conserved. Or, well.

It’s not totally conserved. When the swing moves back and forth, it’s slowing down a bit, by friction and air resistance. Then some of the kinetic energy is converted to heat. That’s why the swing stops after a while. Unless you push it of course.

But then you refill it, with new kinetic energy. So, we can say like this: Mechanical energy in a system is conserved, except that it can be reduced by friction, and it can be changed by outside forces. Here’s a roller coaster. Or as we call it: A darn good example of mechanical energy being conserved. First, we pull the cars up to the top of a hill.

Now they are fully charged with potential energy. Then we let go, and the gravity does its job. Kinetiiiiiiiiiiiic Potentiaaal. Kinetiiic. Potentiaaaaaaaa energy.

Kinetic - potential - kinetic. A roller coaster has just a little friction and air resistance, so the mechanical energy is almost constant. Here’s another experiment. A sling shot. What if Kim releases the slingshot right here?

Oooops. This is also potential energy. But it works a bit differently. The potential energy we saw in the swing, and the roller coaster, depend on how high up those things were. There was a potential to be drawn down by gravity.

So we call it gravitational potential energy. But the potential energy in the sling shot has nothing to do with gravity. Here, the energy is stored in a stretched elastic band. This is elastic potential energy. And this, too, can be converted into kinetic energy.

Aaaaaaaaah! Potential energy Plus kinetic energy Equals mechanical energy.