The force that pulls objects toward Earth and the attractive forces exerted by a body on an object or another object or bodies is called gravity. Gravity causes a ball to fall to the ground after it is thrown into the air. Gravity keeps the Earth in orbit around the sun.
Gravity is a force of attraction between all matter. It is the weakest known force in nature, but it still manages to hold galaxies and the solar system together. The ancient Greek philosophers thought that the motions of the stars and the planets were totally unrelated to events on the Earth. The heavens were the realm of the Gods, where everything existed in perfection. One of these philosophers was called Aristotle. He thought that the stars and planets followed a so-called "natural" motion, and that the force that made them move made contact between them.
Gravity can be measured by using a device called a gravimeter. A metal ball is suspended from a very sensitive coil. Wherever gravity is stronger, it pulls more on the ball, thus stretching the springs. A pointer attached to the spring shows the increase in gravity. Scientists measure gravity because many things depend on it. For example, when launching satellites into space, scientist must know the strength of Earth's gravity so they can determine how fast a satellite must travel to escape the planet's gravity or to remain in orbit around a planet.
The strength of gravity is not the same at all places on Earth. Three things determine the strength of gravity at any given place:
- The distance from the center of Earth
- The spin of the Earth
- The Nearby sources of gravity variations, such as mountains or underground caverns
Consider the distance from the center of Earth. A house at the seashore is at a lower elevation than one in the mountains, which means that it is closer to the center of Earth. The pull of gravity is stronger at the seashore house than it is at the mountain house.
The Earth's spin also produces an effect that can appear to reduce the strength if gravity (though it does not). Known as the centrifugal effect, it is caused by the tendency of a body to move in a straight line unless acted upon by a force trying to change its path. The tendency of a body at the surface of the spinning Earth is to move in a straight line. At the same time, the Earth's gravitational force is pulling the body toward the center of the planet. Part of the Earth's gravitational force is reduced in changing the body's path from a straight line in space into the circle it follows as the earth rotates. This serves to lessen body weight.
Variations in gravity may also be caused by nearby concentrations of mass such as mountain ranges or underground deposits of material. The pull of gravity is greater near large or dense concentrations of mass or deposits of dense materials and is weaker near underground caverns or deposits of light materials, such as oil. Looking for gravity variations with a gravimeter is an important way of searching for deposits of oil or minerals.
Your fuel gauge is below empty. Both engines of the cargo plane you're piloting have sputtered and gone silent. The nose of the plane points down and you begin a terrifying dive toward Earth. In a panic, you make your way out of the cockpit and into the back of the plane where your parachute is stored. A 2,000-kilogram crate is blocking your path. What do you do? No problem! Since the weight of the crate on the plane's floor is actually zero, you would not have to lift it in opposition to gravity or slide it in opposition to its friction with the floor. The force required to overcome inertia of the crate would be small enough to allow you to move it by pushing hard with your foot braced against a wall. How is this so?
Let's look at the crate under normal flight conditions. The weight of the crate pushes down against the floor of the plane. What you might realize is that the floor, which is supported by the airplane's wings and the forces that keep the plane aloft, also pushes up against the crate. It pushes up with a force equal to the weight of the crate, so inside the plane, you're aware of how heavy the crate is.
When your plane goes into free-fall, the crate is still pulled by gravity just as during a normal flight; but the floor is no longer pushing up on the crate, since it and the crate are now falling freely toward the Earth. Gravity is still acting on both the crate and the plane. But inside the airplane, without the upward push from the floor, the crate now seems to be weightless. Both the crate and the pilot will float freely inside the airplane until something like Earth stops them.
Astronauts in orbit experience weightlessness just like objects in the falling aircraft. A space shuttle in orbit is actually in a state of free-fall as it travels around Earth. Hard to imagine? Picture yourself in a small spaceship a few meters above the ground. Now face the setting sun and go in a straight line for about 100 kilometers (62 miles). If you in a perfectly straight line, you should notice that Earth is curving away from you. A shuttle in orbit goes so fast that Earth curves "away" just as much as the shuttle falls. The shuttle falls, but never hits the ground!
Questions to Ponder!
Falling appears to be different for different objects. For instance, which falls faster, a pen or a piece of paper? Why might one fall faster than the other? In real life, when do you experience something like free-fall? For how long? Which falls faster, a one-ton plane or a ten-ton plane?