Gravity is a major player in the study of physical science. It is, of course, the force of gravity that causes objects to fall. One object always exerts a force of attraction on another object. This force of attraction is a pull, like the pull of gravity.
The larger an object is, the greater is the force of its attraction. Consider the fact that the sun, which is much, much larger than the earth, can, even at 90 million miles away, hold the earth and the other eight planets in orbit.
Astronauts who have walked on the moon feel light and weightless because there is very little gravity holding them down. On the other hand, if they were to go to Jupiter, which has much more gravity than the earth, they wouldn't even be able to lift a foot off the ground.
The moon, on the other hand, is much smaller than the earth, and has only about one-sixth of the gravity of the earth.
It was Italian scientist Galileo Galilei who formulated the laws of accelerated motion and free-falling objects. He found that when an object is dropped and falls to the ground it has a falling rate of 9.8 meters per second, squared.
You may wonder, then, why feathers float gently in the breeze instead of falling to the ground quickly, like a brick does.
Well, it's because the air offers much greater resistance to the falling motion of the feather than it does to the brick. The air is actually an upward force of friction, acting against gravity and slowing down the rate at which the feather falls.
The brick, on the other hand, can cut right through the air as if it didn't exist. Galileo discovered that objects that are more dense, or have more mass, fall at a faster rate than less dense objects, due to this air resistance.
A feather and brick dropped together. Air resistance causes the feather to fall more slowly.
If a feather and a brick were dropped together in a vacuum—that is, an area from which all air has been removed—they would fall at the same rate, and hit the ground at the same time.
Understanding these basic facts will help you to be able to answer the question of why some objects fall faster than others.
You can test the rate at which various objects fall, noting both the mass of each object, and how long it takes for it to fall. Be sure to drop all objects from the same height, and be careful to use only objects that can't break. Record all your information in a journal, and chart your results.
Conduct three trials for each object so that you can calculate an average time.
Excerpted from The Complete Idiot's Guide to Science Fair Projects © 2003 by Nancy K. O'Leary and Susan Shelly. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.
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- Science Projects for Beginners: Physical Science
Tools and Materials
- Three feet (three meters) or more of clear, plastic, rigid-walled tube (available at your local plastics store) with an inner diameter of at least 1 inch (2.5 cm)
- Two solid rubber stoppers—one with a hole through it, one without—to fit in the ends of the plastic tube
- Copper tubing about 4 in (10 cm) long that fits tightly in the hole in the rubber stopper (glass tubing can be used—just be especially careful so it doesn't break)
- About six feet (180 cm) of thick-walled, flexible plastic or rubber vacuum tubing
- A coin
- A feather or a small piece of paper
- Vacuum pump (use a regular lab vacuum pump if available; if not, use a small hand pump such as Mityvac®)
- 2 hose clamps
- A partner (preferably an adult)
Assembly
- Insert the solid rubber stopper (the one without the hole) firmly into one end of the plastic tube. Turn the tube so the stopper is on the bottom.
- Put the coin and the feather (or piece of paper) in the tube.
- Push the copper tube through the one-hole stopper and firmly insert the stopper into the open end of the plastic tube.
- Push the vacuum tubing over the copper tube and secure it with a hose clamp, if needed. Attach the other end of the vacuum tubing to the pump; again, use a hose clamp if needed.
- The final assembly should look like the diagram below (click to enlarge).
To Do and Notice
Invert the tube and let the objects fall. Notice that the feather falls much more slowly than the coin.
Now pump the air out of the tube and invert it again (the pump can remain attached while you invert the tube). Notice that the feather falls much more rapidly than before—in fact, it falls almost as fast as the coin.
Let the air back into the tube and repeat the experiment. (Try to avoid rubbing the wall of the tube; otherwise, static electricity may make the feather stick to it.)
What’s Going On?
Galileo predicted that heavy objects and light objects would fall at the same rate. The reason for this is simple. Suppose the coin has 50 times as much mass as the feather. This means that the earth pulls 50 times as hard on the coin as it does on the feather. You might think this would cause the coin to fall faster. But because of the coin's greater mass, it's also much harder to accelerate the coin than the feather—50 times harder, in fact! The two effects exactly cancel out, and the two objects therefore fall with the same acceleration.
This rule holds true only if gravity is the only force acting on the two objects. But if the objects fall through air, then air resistance must also be taken into account.
Larger objects experience more air resistance than smaller objects. Also, the faster an object falls, the more air resistance it encounters. When the retarding force of the air just balances the downward pull of gravity, the object will no longer gain speed; it will have reached what is called its terminal velocity.
Since the feather is so much lighter than the coin, the air resistance on it very quickly builds up to equal the pull of gravity. After that, the feather gains no more speed, but just drifts slowly downward. The heavier coin, meanwhile, must fall much longer before it gathers enough speed so that air resistance will balance the gravitational force on it. The coin quickly pulls away from the feather.
Going Further
The terminal velocity of a human being falling through the air with arms and legs outstretched is about 120 miles per hour (192 kilometers per hour)—slower than a lead balloon, but a good deal faster than a feather!
Components of force make a toy walk and stop at just the right time.
“g,” that’s interesting! Investigating gravity.