Main->Readings->4th Grade Readings->Matter in Motion- >Part 3: Gravity
Vocabulary |
Whenever one object hits another object, or shoves it, or rubs against it, that is called a contact force. A contact force is a force which is applied while touching the object. So far, we have been talking only about contact forces.
There are other kinds of forces that make objects move, however. They happen without any touching at all. It may sound like magic, but it is real. An example of this type of force is magnetism. These forces, called field forces, affect objects even when they are far apart.
Gravity is also a field force. If you stand on the top of a box and then step off into space, you don't stay there. Something pushes you down to the ground. There is nothing touching you, but you still fall. Gravity pulls you from far away.
Gravity is the force between objects made of matter. It is a property of matter. Everything that has mass and takes up space has gravity. It is a very weak force, however, much weaker than magnetism. If it were a strong force, everything in the universe would be stuck together. As it is, the only things with gravity strong enough to make things stick to them are planets and stars. The bigger something is, the more gravity it has.
The Earth's gravity is a steady force. It keeps pulling all the time. Sometimes it pulls on objects but something is in the way, so the objects do not move. For example, if you are sitting on a chair, the gravity is pulling you toward the Earth, but the chair pushes you up and keeps you from falling.
Other times, if nothing is in the way, the Earth's gravity does move objects. This movement is called falling.
Remember that once a force causes an object to start moving, it will keep moving. It does not need any more force to keep it moving in a straight line.
If, instead, you keep applying force, the object will accelerate. It will keep speeding up, faster and faster.
Gravity keeps applying force, so when objects fall, they keep going faster and faster. We say that gravity causes objects to accelerate at about 10 meters per second per second.
That means that in the 1st second, the object goes from zero to 10 meters per second. In the 2nd second, it goes from 10 meters per second to 20 meters per second. By the time it has been falling 5 seconds, it is going 50 meters per second. Pretty fast!
It would keep accelerating forever except for two things. For one thing, air is matter, so the falling object rubs against the air and friction slows it down. For another thing, it has to hit the Earth eventually.
"Wait a minute!" somebody always says. "Don't heavier things fall faster than lighter things?" Feathers and pieces of paper take a long time to fall, not because they are light, but because they have a lot of surface area and they do not move through air easily. If there were no air, everything would fall at the same rate.
An astronaut once took a feather and a hammer to the Moon on the Apollo 15 mission. He held the two objects out in front of him and let go of them at the same moment. They landed on the ground at the same moment. Why? There is no air on the Moon. There is no friction to slow the feather down.
Even before anybody could imagine going to the Moon, somebody figured this out. It was, once again, Galileo. Falling things move very fast and it is hard to see if they fall at different speeds, so instead of dropping them, he put balls on long ramps and watched them roll down more slowly. He found that objects rolled at the same rate no matter what they weighed.
To understand this, suppose you drop a ball that has a mass 10 grams. It will accelerate at about 10 m/s/s.
If you drop another ball that is exactly the same mass, it will do the same thing.
Suppose you drop the two identical balls at the same moment. Will they fall at the same rate and land at the same time? Of course.
'Now suppose you move them closer together, so they are only a centimeter apart. Will they fall at the same rate? Of course. What about a millimeter apart? A nanometer apart?
Now suppose you hold them together in your hand, so they are touching, and drop them at the same time. The fall at the same rate and land at the same time. It does not matter if they are touching or not.
Suppose they are stuck to each other so they cannot be pulled apart. They will fall at the same rate and land at the same time.
Suppose now that you have a ball that has a mass of 20 grams. You drop it. Now, before it hits the ground, think. The new ball has a mass of 20 grams. The two balls you were dropping before had a mass of 10 grams each. Is there any difference between dropping one 20 gram ball and two 10 gram balls?
No. Test it for yourself and see. They fall at the same rate and land after the same amount of time.
Suppose you throw a ball. It comes back down eventually. But if you have ever watched the path of a ball thrown in the air (or a kicked field goal) you notice that it goes up and comes down in a curved path. Newton's First Law says that an object in motion will tend to stay in motion in a straight line unless acted on by an outside force. Why is the ball's path not straight? Because of gravity. You applied a force to the ball when you threw it. While it was traveling, gravity applied a force to it. As it went upwards, gravity slowed it down (decelerated it) and when it reached the top of the curve, gravity made it accelerate downwards, faster and faster. An object which shoots through the air (or through space) is called a projectile. The study of projectiles is called ballistics.
Anyone who has played catch can figure out where the ball is going to land when someone throws it. It is harder, though, to figure out where a rocket would land when it is shot into the air, or where a bullet will end up when it is fired from a gun. However, scientists can calculate the path of a rocket so well that they have landed human beings on the Moon and sent probes to other planets in our solar system.
In this section, you learned:
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
Return to Table of Contents
Go on to Part 4: Putting Energy to Work
This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org