If you throw a ball into the air, it rises then falls to the Earth. If you throw it straight ahead, it begins curving toward the Earth and finally falls. If you hold the ball in your hand and drop it, it falls again. In every case, the ball falls to the ground. The reason the ball always falls is gravity.
Gravity is a force — a push or a pull. Every time you drop a ball from your hand or throw it upward or straight ahead, it is pulled back to the Earth. When an event in nature occurs repeatedly in the same way, we say that it takes place according to natural law. Since the ball or any object is always pulled to the Earth by gravity, the force of gravity acts according to a natural law that is called the law of gravity.
The Discovery of Gravity
The first person to work out the law of gravity was English scientist and mathematician Isaac Newton (1642-1727), during his studies of the motions of objects. Newton was puzzled about the motion of the moon and wondered why the moon did not just fly off into space; what force kept it in its orbit, or curved path, around the Earth.
It is said that Newton's study of motion was influenced by his observation of an apple falling from a tree. This and similar events helped him determine that the Earth appeared to be pulling all objects to itself and that the force that pulled the apple to the ground must have been the same one that kept the moon in its orbit around the Earth. Newton went on to discover that this force was indeed universal.
Using mathematics, Newton determined that the moon's orbit is the result of two different motions. One is its movement along a straight line in space. This, according to Newton, is the motion an object will follow if it is not acted upon by any forces other than the one that put it in motion. The other is acceleration, a change in the direction or speed of a moving object. In the case of the moon, its movement in a straight line is being constantly changed to a movement that curves toward the Earth. The combination of the two motions, which are happening at the same time, causes the moon to move in a curved path around the Earth. Newton concluded that gravity is what pulls the moon out of a straight path and keeps it in orbit around the Earth.
Newton also determined that everybody in the universe must have gravity and that every body pulls on every other body. ("Body" is the word scientists use for an object in space.) His idea that every body possesses gravity is known as the theory of universal gravitation.
The words "gravity" and "gravitation" are both used by scientists to describe the pull of one object upon another. They usually use "gravitation" to refer to the idea that every object in the universe attracts every other object. They use "gravity" when speaking of this attraction at the surface of bodies such as the Earth.
Newton used the idea of gravitation to explain how the planets stay in orbit around the sun. All the planets orbiting the sun would fly off into space if some force did not pull them toward the sun. Newton reasoned that the planets are held in their orbits by the same force that holds the moon in its orbit — the force of gravitation.
The Strength of Gravitation
Newton found that the strength of gravitation depends on two things. First, it depends on the mass, or amount of matter, that a body contains. A body with a large amount of mass has a stronger gravitational force than a body with a small amount of mass. Because the Earth's mass is greater than the moon's, its gravitational pull on us is greater than the moon's gravitational pull would be on us if we stood on the moon. For this reason we weigh more on Earth than we would on the moon.
Second, the strength of gravitation depends on the distance between the bodies. The force of gravitation is strong between bodies that are close together, and it is weak between bodies that are far apart.
Newton worked out a mathematical equation to determine the force of gravitation between two bodies. The equation is
F=G X m1X m2/r2
In this equation, f stands for the force of gravitation. The mass of the first body is m1, the mass of the second body is m2, and r stands for the distance between the two bodies. The quantity G stands for Newton's constant of gravitation, a number that is needed to determine the exact force between the two bodies. This equation shows that the amount of mass in each body and the length of the distance between them determine the strength of the gravitational force they exert on one another. When the distance between the two bodies is increased, the force of gravitation between them lessens. If the two bodies are pulled closer together by gravity, the force of gravitation between them increases.
How does Newton's law of gravitation apply to an object, such as a ball, that is dropped near the Earth's surface? Because of the Earth's size, the change in the gravitational force experienced by a ball falling a few hundred yards, or even a mile, is hardly noticeable. Thus, the force of gravity can be considered as constant.
This constant force, however, causes the ball to accelerate, or to fall faster and faster. The acceleration near the surface of the Earth is 32 feet(9.8 meters) per second per second. This means that the ball's speed increases by 32 feet (9.8 meters) per second each second that it falls. Just before the ball begins its fall, its speed is zero. One second later it is falling at a speed of 32 feet (9.8 meters) per second. After two seconds of falling, the ball's speed is 32 plus 32, or 64 feet (19.6 meters) per second. Thus, a body that falls from a great height accelerates to a great speed and strikes the Earth with great force. That is why an object is more likely to break if it falls from a great height than if it falls from only a low height. Any object falling in a vacuum, whether it is heavy or light, accelerates at the same rate — 32 feet (9.8 meters) per second per second — and will keep on accelerating. A body falling in air, however, is slowed down by air resistance. The amount that it slows down depends on its size and shape. Thus, in a vacuum, a feather and a marble fall with the same acceleration. When falling in air, however, the large surface of the feather will produce more friction, causing it to fall more slowly than the marble. As the speed of a falling body increases, air resistance also increases. If the force of air resistance becomes equal to the force on the body caused by gravity, the body will no longer increase its speed as it continues to fall. It will have reached its maximum speed, which is called its terminal velocity.
Gravity can be measured by using a device known as a gravimeter,in which a metal ball is suspended from a very sensitive spring coil. Wherever gravity is stronger, it pulls more on the ball, thus stretching the spring. 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, scientists must know the strength of the 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 the planet.
The Earth's Gravity
The strength of gravity is not the same at all places on the Earth. Three things determine the strength of gravity at any given place: (1) the distance from the center of the Earth, (2) the spin of the Earth, and (3) the nearby sources of gravity variations, such as mountains or underground caverns.
Consider the distance from the center of the 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 the Earth. The strength of gravity is stronger at the seaside house than the strength of gravity is at the mountain house.
The Earth's spin also produces an effect that can appear to reduce the strength of gravity. 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 outward 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 actually follows as the Earth rotates, and this serves to lessen the body's weight.
An example is a body at the Earth's equator, where the centrifugal effect is greater than anywhere else on the surface of the planet. A body at the equator must travel nearly 24,000 miles (39,000 kilometers) during one rotation of the planet, but this distance and the centrifugal effect decrease as you move away from the equator and toward the poles. The result is that the weight of a body on the Earth's surface increases slightly as it moves away from the equator and toward the poles. This is because the Earth's gravitational force is slightly less at the equator than at the poles. A bag of sugar at the equator would weigh about 1/1,000 less than it would in Honolulu, Hawaii, which is closer to the North Pole.
Variations in gravity may also be caused by nearby concentrations of mass such as mountain ranges or underground deposits of materials. The pull of gravity is greater near large or dense concentrations of mass or deposits of dense materials, and it 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.
.Mass and Weight
It is important to distinguish between mass and weight. Remember that mass is the amount of matter in a body. The mass of a body remains the same wherever the body may be. For example, the mass of an astronaut is the same whether the astronaut is on the Earth, the moon, or any other place in the universe. Weight is a measure of the pull of gravity on a body. Therefore, the weight of a body can vary depending upon the pull of gravity on it. For example, if the Earth's gravity pulls on an astronaut with a force of 154 pounds (70kilograms), the astronaut's weight is 154 pounds. Suppose, however, that this same astronaut visits the moon. The moon has less mass than the Earth, so its pull of gravity is less. On the moon, the astronaut's weight would only be about 26 pounds (12 kilograms). Although weight may change from place to place, mass remains the same.
Weightlessness and Acceleration
Suppose a person and a scale are falling freely somewhere within the Earth's gravitational field. If the person is standing on the scale, the arrow on the scale will point to zero, indicating that the person weighs nothing. This is because both the person and the scale are experiencing the same acceleration. This effect takes place within a spacecraft. Inside a freely falling spacecraft, all objects are experiencing the same acceleration. The result is that to the people in the spacecraft, gravity appears to have disappeared, and all of the objects in the spacecraft, including themselves, seem to be weightless.
Now imagine a spacecraft in empty space far from the gravitational pull of any planet or star. If the spacecraft's rockets began to blast, the astronauts inside would feel a force pressing them toward the floor, and they would be unable to distinguish this from the force of gravity. The scientist Albert Einstein (1879-1955) was the first person to point out that the effects of gravitation and acceleration are the same. He called this idea the principle of equivalence.
In his general theory of relativity, Einstein also proposed that gravity was not a force that pulled bodies in a straight line toward one another but instead was the result of a curving or warping of space and time. A body that moves in the vicinity of another body such as the sun bends to follow the curves in space and time created by the gravitational influence of the sun. This, according to Einstein, produces the orbits of planets, not a force acting between the sun and the planets as Newton believed. Building upon this idea, Einstein developed a mathematical theory of gravity that made some remarkable predictions.
One of Einstein's predictions was that rays of light should be bent by the curved space around a massive body such as the sun as the light rays come near it. This prediction has been verified by scientists. Another prediction was the existence of black holes. In black holes, the warping of space and time is so strong that anything unlucky enough to fall inside one can never escape, not even light itself. Although astronomers cannot "see" black holes, there is increasing evidence that they exist. Einstein’s theory also predicts that gravitational waves should be produced when two massive bodies such as stars orbit one another. Some evidence for gravitational waves has been discovered by scientists studying a binary pulsar, a swiftly rotating neutron star orbiting a companion star.