A physics project from Science Buddies

Key Concepts Physics Gravity Potential energy Kinetic energy Friction Conservation of energy

How much energy does a roller coaster need to go through a loop without getting stuck? Build your own marble roller coaster in this project and find out!

Roller coasters rely on two types of energy to operate: gravitational potential energy and kinetic energy. Gravitational potential energy is the energy an object has stored because of its mass and its height off the ground. Kinetic energy is the energy an object has because of its mass and its velocity.

When a roller-coaster car reaches the very top of its first big hill it has a lot of potential energy because it is very high off the ground. It moves over the top of the hill very slowly, so it has almost no kinetic energy. Then it drops down the other side of the hill and starts going very fast as its height rapidly decreases. The potential energy is converted to kinetic energy. This process repeats as the car goes through hills, loops, twists and turns. Whenever it goes up it gains more potential energy with height but loses kinetic energy as it slows down. Energy is never created or destroyed—it just converts from one form to another. This principle is known as conservation of energy.

We know from experience, however, that a roller coaster doesn't keep going forever. Eventually it slows down because of friction (a combination of air resistance and contact with the track). If energy isn't created or destroyed, where does that energy go? It is converted into heat. This is why you can rub your hands together to warm them up—friction converts energy from your moving hands into heat!

Does conservation of energy restrict a roller coaster's movement? For example, can a roller coaster ever go through a loop that is taller than its initial hill? Try this project to find out!

You should have found that the marble had to start higher than the top of the loop in order to make it the whole way through the loop. This happens because some energy is always lost to friction as the marble rolls down the track. You need to start the marble higher than the top of the loop so it has enough extra energy to get the whole way through the loop without stopping.

If you watch the marble closely, you might be able to see that it is going the fastest right at the bottom of the hill before it enters the loop. As the marble rolls down the hill its potential energy is converted to kinetic energy (its height decreases, but its velocity increases). When the marble goes back up the loop its height increases again and its velocity decreases, changing kinetic energy into potential energy. If you added a straight piece of track at the bottom of your loop, you could observe how the marble gradually rolled to a stop due to friction.

The more features you add to your track, the more initial potential energy the marble will need to make it through all of them without stopping. You might notice that the pipe insulation flexes and bends as the marble zips around—this can also cause the marble to lose some energy (it takes energy to bend the insulation). Making your track more rigid by taping it to supports (such as boxes or pieces of furniture) will help avoid this type of energy loss, allowing your marble to go farther.

Paper Roller Coasters, from Scientific American Marble Roller Coaster: How Much Height to Loop the Loop? from Science Buddies Rolling Race, from Scientific American STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Ben Finio is a senior staff scientist at Science Buddies and a lecturer at the Cornell University Sibley School of Mechanical and Aerospace Engineering. Follow him on Twitter @BenFinio.

Thomas Frank and E&E News

Chelsea Harvey and E&E News

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