Ever notice that a conventional roller coaster never goes higher than its first hill? Roller coasters always have to end lower than where they started because of a fundamental idea called the conservation of energy—energy is never created or destroyed, it only changes forms. Put another way, you’ll never end up with more energy than you start with.
In this project, you’re going to explore the conservation of energy. Rather than build a roller coaster, though, you’re going to use a simple pendulum as a stand-in. And, just to test your intuition, you’re even going to place the safety of your nose in the hands of physics!
- Bowling ball, dumbbell, concrete block, or other large mass that can be supported by a rope
- Rope that is strong enough to hold the mass
- Ceiling beam, tree limb, or anything else sturdy enough to support both the rope and mass
- Securely tie the rope to the ceiling beam (or whatever else you’re using); tie it high enough so the rope can swing freely without hitting the ground.
- Tie the other end to the mass. Give it a few tugs to make sure it’s good and secure!
- Taking hold of the mass, step back until you can touch the mass to the tip of your nose while keeping the rope taut.
- Now for the fun part: let go of the mass! Don’t push the mass, just let it go!
- Stand perfectly still while the mass swings away and starts to swing back towards you. Didyou need to jump out of the way? Did it touch your nose again? Roughly how close did it come to your face?
- Let it swing back and forth half a dozen times or so. Notice how close it comes to your face each time. What changes do you notice? Does the distance to your face stay the same? Does it change?
*Note: If you really don’t trust physics just yet, replace yourself with a vase, a book, even a wall. Once you’ve tried it a couple of times with something else, try jumping in and seeing if you can keep yourself from flinching!
Assuming you don’t add any acceleration to the ball and keep perfectly still, the mass will not touch your face. With every swing, it will get farther and farther away. Eventually, it will just stop.
When you release the weight, it has all the energy it’s going to have. Specifically, it’s loaded up with a form of energy called potential energy, or stored energy. As soon as you let go, gravity pulls the weight down and the potential energy starts converting to another type called kinetic energy, the energy of motion. At the bottom of the swing, all potential energy is fully transformed into kinetic energy. As it climbs up the other side, the kinetic energy starts changing back into potential energy. This repeats over and over: potential turns to kinetic, kinetic turns to potential.
But the transformation between potential and kinetic energy isn’t perfect. Friction from the air and the rope steals some of the energy during every swing. That energy goes into heating up—very slightly—the air, the rope, and the support beam. When the weight is coming back to your face, it has less energy than it started with and can’t climb as high. Your nose is perfectly safe!
That’s why it’s important not to push the ball away. When you do that, you’re adding more energy to the pendulum. If you add more energy than friction takes away, it can climb higher than its starting point and…whack!
The conservation of energy is also why roller coasters can’t go higher than their starting point. After being dragged up the first hill, they have all the energy they’re going to have—just like the weight right before you let go. Every hill after that has to be lower because it doesn’t have enough energy to go higher. The only way around that is to carry it up another hill and let go again; the same thing will happen if you catch the bowling ball in mid-swing and pull it back up to your nose.
Try releasing the weight from different heights and see if that has any effect. Try releasing it with a push and see what happens—though don’t use your nose for that test. Put something else in the way that can be easily knocked over without breaking (a book standing up on the edge of a table, for example). What happens now? If the conservation of energy is true, where does the energy come from that lets the mass climb higher?