The Energy Bank


Overview


Did you know that if you dropped a bowling ball from the top of the Empire State Building, it would be going more than 200 miles per hour by the time it hits the ground!? That would leave a pretty big hole in the ground! What makes it so that a normal bowling ball can blast a hole in the ground in a split-second? It's all about energy.

Energy comes in a lot of different forms. When you think of energy, you might think of things that you use every day at home. Yes, energy is used to power the light bulbs, refrigerator, microwave, and every other electric appliance in your house. Energy is also in the gasoline that powers your car. Energy is in the food you eat. Electricity, gas, and food are all substances that contain energy, but it's a little hard for us to actually see the energy in them.

There are places where we can see energy at work, though. In fact, any time you see an object moving, you are seeing energy. The energy in a moving object is called kinetic energy. The energy that allowed our bowling ball to blast a hole in the ground was kinetic energy. Another type of energy you can see is called potential energy. Anytime an object is lifted off the ground, it has potential energy. Potential energy is energy that has the potential to be changed to another type of energy. For instance, if you have a bowling ball in your hands and you're standing on the top of the Empire State Building, the bowling ball has potential energy. If you drop it, it will start to go faster and faster, and the potential energy will be changed to kinetic energy (the type of energy that all moving objects have).

In this activity, we're going to explore the relationship between potential energy and kinetic energy. No, we're not going to drop anything off the roof! Instead, we'll build an Energy Bank, where you can come to exchange potential energy for kinetic energy or vice-versa, all before your very eyes. The Energy Bank has two devices: the first one is a pendulum, just like the kind that you see in an old clock. We'll just call this "The Pendulum." The second type will look more like a flying saucer, so we'll call it "The UFO."

Setup

Here's what you'll need:
9 long tubes
7 short tubes
8 five-way joints
3 cross-beam connectors
2 ball drops
1 accessory connector
1 catch basket
1 vortex
3 support arms
9 short support clips
4 wide, 90-degree turns
balls
thread or string

Assemble the structure shown below.

Now make the Pendulum like this:

The UFO is a little trickier to make. First, slide three short support clips onto each support arm, and snap the support arms onto the bottom of the vortex like this:

Now snap together the four wide, 90-degree turns to make a circle. Then just snap the circle onto the support arms like this:

Okay, now let's put the Pendulum in its place in the Energy Bank. It's pretty easy to set up. You just rest the Pendulum in the ball drops like this:

Now for the UFO. Just slide it onto the long tube (you'll have to undo the long tube so you can slide the UFO onto it).

In order to make the UFO "fly," tie a thread to the circular track, loop it up around the five-way joint, and tie it to the other side of the circular track. Make sure the UFO is hanging by the thread, not resting on the bottom of the frame.

Stage I

First, let's just watch how the Pendulum works. Put some marbles into the basket and start the Pendulum swinging. The marbles help the Pendulum swing smoother, but they're not necessary. You might want to try removing the marbles and swinging the pendulum again. Does it swing for as long a time? What makes the Pendulum eventually stop swinging?

Now look really closely at what's happening. At what point in its swing is the Pendulum moving the fastest? At what point does it stop moving (just for a split-second)? At what point is the Pendulum the highest off the ground? Where is it the lowest?

Pendulums are used in clocks because they have one very special quality: it doesn't matter how far they swing, they will always swing the same amount of times per minute! You can prove it by saying "tick-tock" as the Pendulum swings. Ask a friend to listen to you for a little while and then tell you if your "tick-tocking" ever got faster or slower. Chances are, you'll be tick-tocking at exactly the same speed, no matter how far the Pendulum swings! The only way to change how fast it tick-tocks is to move the catch basket up or down on the tube. Try it out. When the basket is near the top of the tube, does the Pendulum tick-tock faster or slower?

Now let's try out the UFO. To make the UFO fly, just spin it around until the thread winds around the tube all the way. Now let it go! It should spin one way, then spin the other, and so on until it comes to a rest. Does it remind you of the Pendulum at all?

Ask yourself the same questions that you asked about the pendulum. At what point is the UFO moving the fastest? At what point is it not moving at all? At what point is the UFO the highest off the ground? Where is it the lowest?

Remember how the Pendulum would "tick-tock" at the same speed regardless of how far it swung? Do you think the UFO will do the same thing? Try it out. Say "tick" as the UFO starts spinning one way and "tock" as it starts spinning the other way. Have your friend listen closely to see if your tick-tocking gets faster or slower. What happened? Would you want to buy a clock that used the UFO to keep time?

Explanation

Both the Pendulum and the UFO work by changing potential energy into kinetic energy and back again. When they are the highest off the ground, they have a lot of potential energy. Then, as they start to swing or spin towards the ground, they speed up. The potential energy is being changed to kinetic energy. At their lowest point, they have no potential energy left, but they sure do have lots of kinetic energy! This kinetic energy is changed back to potential energy as they start getting higher again and slowing down. Then they come to a stop, and the cycle starts over again.

One of the neat things about the world that we live in is that you can never create or destroy energy. This principle is called Conservation of Energy. The Pendulum and the UFO are a perfect example of this. As you watch them move, you see that whenever there's a lot of potential energy, there isn't much kinetic energy. And whenever there's a lot of kinetic energy, there isn't much potential energy. The two types of energy always balance out! You'll never see a lot of potential energy and kinetic energy at the same time.

So energy is never destroyed or created, right? It can only change from one form to another. But if that's true, why do the Pendulum and the UFO stop after a while? When they stop, you know they don't have any potential energy left because they always stop at the lowest position. And you know they don't have any kinetic energy left because they're not moving. So where did all the energy go? What actually happened is that the energy got changed to heat. The plastic pieces rubbing against each other actually got a little bit hotter, although you can't feel it because they didn't get hotter by a noticeable amount. But the energy was still there.

As for why the Pendulum "tick-tocked" at the same speed while the UFO didn't, don't worry about that. It would take some pretty difficult math to explain it. But now at least you know why pendulums are used in clocks!

Exercises

Stage I

1. Name two things that you might see on a playground that change potential energy into kinetic energy. (a slide and a swing).

2. Speedy Steve takes his friends out for a ride in his brand new sports car, which goes 100 miles per hour. His enemy, Eagle Eddie, is put-putting around in his airplane high overhead. Eagle Eddie sure has a nice view from above, but his airplane is only going 50 miles per hour. Who has more kinetic energy: Speedy Steve or Eagle Eddie? Who has more potential energy?

3. Now Eagle Eddie looks down at Speedy Steve and his friends. Thanks to his eagle eyes, he can see Steve and his friends pointing their fingers at him and laughing! They obviously think it's funny that Eagle Eddie's airplane is only going 50 miles per hour. Eagle Eddie thinks they'll stop laughing if he can get his airplane to go faster than Speedy Steve's car. His engine is already at full power, but he thinks there must be another way to go faster. What should Eagle Eddie do?