Bring Science Home A physics project from Science Buddies
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
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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 This activity brought to you in partnership with Science Buddies Discover world-changing science. Explore our digital archive back to 1845, including articles by more than 150 Nobel Prize winners. Subscribe Now!The purpose of the coaster's initial ascent is to build up a sort of reservoir of potential energy. The concept of potential energy, often referred to as energy of position, is very simple: As the coaster gets higher in the air, gravity can pull it down a greater distance. You experience this phenomenon all the time. Think about driving your car, riding your bike or pulling your sled to the top of a big hill. The potential energy you build going up the hill can be released as kinetic energy — the energy of motion that takes you down the hill. Once you start cruising down that first hill, gravity takes over and all the built-up potential energy changes to kinetic energy. Gravity applies a constant downward force on the cars. The coaster tracks serve to channel this force — they control the way the coaster cars fall. If the tracks slope down, gravity pulls the front of the car toward the ground, so it accelerates. If the tracks tilt up, gravity applies a downward force on the back of the coaster, so it decelerates. Since an object in motion tends to stay in motion (Newton's first law of motion), the coaster car will maintain a forward velocity even when it is moving up the track, opposite the force of gravity. When the coaster ascends one of the smaller hills that follows the initial lift hill, its kinetic energy changes back to potential energy. In this way, the course of the track is constantly converting energy from kinetic to potential and back again. This fluctuation in acceleration is what makes roller coasters so much fun. In most roller coasters, the hills decrease in height as the train moves along the track. This is necessary because the total energy reservoir built up in the lift hill is gradually lost to friction between the train and the track, as well as between the train and the air. When the train coasts to the end of the track, the energy reservoir is almost completely empty. At this point, the train either comes to a stop or is sent up the lift hill for another ride. At its most basic level, this is all a roller coaster is — a machine that uses gravity and inertia to send a train along a winding track. Next, we'll look at the various sensations you feel during a roller coaster ride, what causes them and why they're so enjoyable. |