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

Introduction

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!

Background

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!

Materials

• Foam pipe insulation (1.5 inches in diameter, at least 6 feet in length—or more if you would like to eventually add more features to your roller coaster)
• At least one glass marble (or other small heavy ball that will roll easily through the foam insulation, such as a metal ball bearing)
• Utility knife
• Table or chair

Preparation

• Ask an adult to use the utility knife to cut the pipe insulation in half lengthwise, forming two U-shaped channels.

Procedure

• Curl one end of a piece of pipe insulation into a loop, roughly 1 foot in diameter.
• Use masking tape to hold the loop in place and tape it to the floor on both sides of the loop. Make sure tape is not blocking the inside of the channel (it's okay to have tape on the inside, just make sure it is pressed flat against the foam and will not block the marble).
• Tape the free end of the pipe insulation to a table or chair, forming a large hill leading down to the loop.
• Place your marble a few inches from the bottom of the hill and release it. Does the marble make it through the loop?
• Move your marble a few inches up the track and release it again. Keep repeating this process until the marble goes the whole way through the loop. How high does the starting position need to be before the marble goes through the loop? Is it lower, the same height or higher than the top of the loop?
• If you need to make your hill higher, tape the two pieces of pipe insulation together end-to-end, and keep trying from greater heights.
• Can you describe how energy is changing throughout your marble's journey down the "coaster"?
• Extra: Add other features to your roller coaster, such as twists, turns and spirals. How high does the hill need to be for the marble to make it through all the features without stopping?
• Extra: Watch your marble closely and observe its velocity. Where is the marble going the fastest? Where is it going the slowest?
• Extra: Add a straight piece of track to the end of your roller coaster at the bottom of the loop. How far does the marble roll before friction brings it to a stop?

Observations and Results

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.

More to Explore

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