It is widely known that a free-falling object is an object which is falling under the sole influence of gravity; such an object has an acceleration of 9.8 m/s/s, downward (on Earth).This numerical value for the acceleration of a free-falling object is such an important value that it is given a special name. It is known as the acceleration of gravity - the acceleration for any object moving under the sole influence of gravity. A matter of fact, this quantity known as the acceleration of gravity is such an important quantity that physicists have a special symbol to denote it - the symbol g. The numerical value for the acceleration of gravity is most accurately known as 9.8 m/s/s. There are slight variations in this numerical value (to the second decimal place) which are dependent primarily upon on altitude. We will frequently use the approximated value of 10 m/s/s in The Physics Classroom in order to reduce the complexity of the many mathematical tasks which we will perform with this number. By so doing, we will be able to better focus on the conceptual nature of physics without too much of a sacrifice in numerical accuracy. When the moment arises that we need to be accurate (such as in lab work), we will use the more accurate value of 9.8 m/s/s.
g = 10 m/s/s, downward
Gravity is really an unknown force. We can define it as a field of influence, and that it effects the entire existence of the universe. Some people think that gravity consists of particles called gravitons, which travel at the speed of light. The only thing we do know is how gravity operates in different parts of our universe. Without gravity, there would be no space and time. There is a legend that says that Galileo once dropped two objects off the Leaning Tower of Pisa to show that the heavier of the two objects dropped faster. If a feather and hammer were the two objects he used then obviously the hammer would hit the ground first. This is due to air resistance, which is the force air exerts on a moving object. This force acts in the opposite direction to that of the object's motion. In the case of a falling object, air resistance pushes up as gravity pulls down, which causes the object to slow down. When Galileo's experiment was repeated on the moon, the hammer and the feather hit the ground at the exact same time. This is due to the fact that the moon has no atmosphere. Therefore, air resistance doesn't exist on the moon. Also, the amount of air resistance on an object depends on the speed, size, shape, and density of the object. The larger the surface area of the object, the greater the amount of air resistance on it. This is why feathers, leaves, and sheets of paper fall more slowly than pennies, acorns, and crumpled balls of paper. There is another legend that states that when Newton was lying against a tree in an orchard, he was struck on the head by an apple. He wondered what provided the acceleration for the apple to fall to the ground. Was this a force of the earth on the apple? If so, then the apple must exert a force on the earth according to Newton’s law of action/reaction forces. Newton applied this theory unto the planets, which orbit the sun. He found by studying astronomical data, that the force that held the earth in orbit around the sun was the same force that drew the apple toward the earth.
Every planet has mass and so every planet exerts a gravitational force on nearby objects. We say that planets have gravity. However, what we really mean is that there is a gravitational force of attraction between the planet and a person standing on the planet's surface. This force depends on the visitor's mass, the planet's mass, and the planet's radius. Accordingly, people have different weights on different planets. For example, a person on the moon weighs only about 1/6 as much as on earth. The moon's radius is 25% earth's radius and the moon's mass is 8% of earth's mass. So, if a student weighs 150 pounds on earth, they would weigh only (1/6) * 150 pounds, which equals 25 pounds, on the moon. Gravity does more than just keeping planets orbiting the sun and causing people to have weight, gravity also causes tides. In simple terms, the tides are caused by the gravitational attraction between the moon and earth's oceans and by the motion of earth through outer space.
There is gravity everywhere. It gives shape to the orbits of the planets, the solar system, and even galaxies. Gravity from the Sun reaches throughout the solar system and beyond, keeping the planets in their orbits. Gravity from Earth keeps the Moon and human-made satellites in orbit. It is true that gravity decreases with distance, so it is possible to be far away from a planet or star and feel less gravity. But that doesn't account for the weightless feeling that astronauts experience in space. The reason that astronauts feel weightless actually has to do with their position compared to their spaceship. We feel weight on Earth because gravity is pulling us down, while the floor and/or ground stop us from falling. We are pressed against it. Any ship in orbit around the Earth is falling slowly to Earth. Since the ship and the astronauts are falling at the same speed, the astronauts don't press against anything, so they feel weightless. You can feel something very like what the astronauts feel for a moment in a fast-moving elevator going down or in a roller coaster, when you start going down a big hill. You are going down rapidly, but so is the roller coaster or the elevator so for a second you feel weightless.
Gravity is a very complicated subject, but scientists are learning more and more about it as time goes on. Contributions from people such as Newton and Einstein helped shape the way we see things today. Without them, no telling what kind of misconceptions we all might believe in today.
BIBLIOGRAPHY
- Encyclopedia Britannica (Section on Gravitation)
- The World Book Encyclopedia (Section on Gravity)
