Ever seen something that looked impossible? In this science fair project, we’re going to investigate the physics of torque, balance, and the center of gravity using a “discrepant event”—something that appears to defy the same laws of physics that make it work in the first place. Grab a ruler, a hammer, some tape, and a piece of string, and get ready to make something very cool.
How can you balance a ruler on the very edge of a table so that only a quarter of an inch of the ruler is touching the table?
- First, pick up your ruler. Can you balance it on your index finger? Try using the six inch mark on the ruler as your balancing point. You’ve now identified the ruler’s center of gravity, the place where the ruler’s mass is distributed equally on either side of a fulcrum—a fixed point about which another object can rotate. In this case, the fulcrum is your finger. You’ll notice that on our ruler, the center of gravity should be right in the middle.
- Now, take your ruler and lay it on a table or desk. How much of the ruler can you nudge over the edge of the table before the ruler falls off? You should find that the ruler falls over the edge of the table once you nudge the ruler’s center of gravity over the table’s edge. This is because you’ve caused the mass to be unequally distributed over our new fulcrum (the edge of the table).
- Now, we’re going to try to identify where the center of gravity on our hammer is. Holding the hammer lengthwise, can you balance it by placing your finger under the middle of the hammer? Probably not! In fact, you should find that the hammer’s head is heavier than the hammer’s handle. We can conclude that the center of gravity is somewhere closer to the hammer’s head.
- How can we use the hammer to balance the ruler on the edge of the table so that only a quarter of an inch of the ruler is touching the table? For this challenge, you’re not allowed to put the hammer on top of the ruler! Here’s a hint: when you put two objects together, the resulting object has a new center of gravity. Illustrate what you think the thing we’re going to build might look like. This will be your hypothesis.
- Now, it’s time to build our device. Take your string and make a loop about 3 inches in diameter.
- Loop your string over your hammer and tape it so that the string is secured somewhere near the middle of the hammer.
- Now, loop your string over your ruler. This end of your loop should be at about the two inch mark of the ruler.
- The end of the hammer’s handle should intersect and form an acute angle with the ruler at the ruler's eight inch mark.
- Now, place the end of the ruler that starts with zero at the edge of the table. If you did it right, the whole thing should balance, and you should even be able to nudge the ruler closer and closer to the edge of the table without it falling to the ground!
- If your hammer and ruler don’t balance on the edge of the table, try adjusting the position of the hammer relative to the ruler. What do you notice about where the hammer’s head is relative to the fulcrum (the edge of the table)? How close can you get the hammer head to the fulcrum without making everything fall over?
You should have gotten everything to balance nicely, as long as the hammer’s head was hanging somewhere underneath the table.
Think about kids on a seesaw. The plank on a seesaw is a lot like our ruler! When it has nobody on it, the center of gravity is smack-dab in the middle, right where the fulcrum is located. Now, if two equally heavy children sat at either end of the seesaw, would anything change? Not really. The center of gravity stays the same, because the seesaw’s mass is still equally distributed on either side of the fulcrum.
What happens when a heavier kid sits on one end of the seesaw? The center of gravity is no longer directly above the fulcrum, so more torque, or force, is applied to one side of the seesaw than the other. The weight of the heavier child causes his end of the seesaw to sink to the ground, while the lighter child is lifted up into the air.
This is very similar to what we saw happen with our hammer and our ruler—but instead of applying weight to the top of a seesaw like a child would, the hammer’s center of gravity pulls on the ruler from underneath the table. In fact, as long as the center of gravity (the hammer’s head) stays somewhere underneath the table (and on the table side of the fulcrum), the ruler remains perfectly balanced. If you used a plastic ruler for this experiment, you might see the ruler getting flexed up into the air because of torque being applied through the hammer’s handle!
Now that you’ve learned how center of gravity helps objects balance, what other things can you build that can balance on the edge of a table? Keep what you’ve learned about torque, balance, and center of gravity in mind as you explore.