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. This was the
force of gravity that is given by this scary formula:
F = Gm1*Gm2
F equals the force in Newtons, G equals the gravitational constant which is 6.67 * 10^-11 Nm^2 | kg^2, m1 and m2 equal the mass of each body in kilograms, and finally, r equals the distance between the 2 bodies in meters.
If all of this is confusing, I feel your pain, because it took me a long time to get this all down!
Another concept that is important to understand is terminal velocity. Terminal velocity is the highest velocity that will be reached by a falling object. As an object falls through air, air resistance gradually increases until it balances the pull of gravity. According to the law of inertia, when the forces acting on an object are balanced, the motion of the object will not change. When this happens, the falling object will stop accelerating. It will continue to fall, but at a constant, final velocity.
Newton’s laws of motion and law of gravitation can be used to explain the forces, position and motion of all objects in the universe. A simple
analogy of how gravity controls the motion of a planet around the Sun can be shown by a mass on the end of a string being spun around in a horizontal
plane at constant speed. The ball has constant speed but the direction is always changing so according to the definition of velocity the object must be undergoing a constant acceleration. According to Newton s second law, for a mass to be accelerating, it must have a resulting force acting upon it. The question is: Where does this force come?
The forces involved can be examined by considering what happens when the string breaks.
When the string breaks the mass is no longer constrained to travel in its circular orbit and moved off in the direction as shown. This indicates that there must be a force holding the mass in its circular orbit. It is directed towards the center of the circle and is called the centripetal force. The centripetal force has a resulting centripetal acceleration. The thing is, you can extend all of these concepts, and apply them onto the objects like the sun, moon, planets, and even entire galaxies. The gravitational force of the sun, acting on the earth, keeps the earth in its orbit, preventing it from traveling away into interstellar space. The gravitational force of the earth, acting on us, holds us to the earth’s surface. The gravitational attraction between a person
and the earth is proportional the person’s mass and inversely proportional to the square of the planet’s radius (distance from the person to the center). This number for gravitational attraction is called your weight.
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.
Einstein predicted gravitational waves. They are best understood in comparison with electromagnetic waves, which were predicted by Maxwell in 1864 and discovered by Hertz 22 years later.
Hertz discovers electromagnetic waves in 1886. Electromagnetic waves are waves of electricity. They give us our sense of vision with which to see the universe. Gravitational waves are waves of gravity. They are vibrations of space itself. They travel through space at the speed of light, but are more like sound than light.
Hertz’ discovery set the foundation for the electronic revolution of the twentieth century. Electromagnetic waves not only revolutionized our lives, but also our knowledge of the universe. Astronomers gradually opened the electromagnetic spectrum, first using visible light and then radio, x-rays and gamma rays. Each new part of the spectrum provided us with dramatic new insights into the universe.
Einstein predicted gravitational waves in 1916. They have not yet been directly detected on earth, although astronomers Joe Taylor and Russell Hulse received the 1993 Nobel Prize for proof of their existence, by showing that a
star system is losing energy by producing gravitational waves.
Gravitational waves are a completely new spectrum. If electromagnetic waves let us see the universe, gravitational waves will let us hear the universe. They will provide us with a new sense, the sense of hearing, with which to explore the universe.
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.