Gravity is really an unknown force. Wecan define it as a field of influence, andthat it effects the entire existence of theuniverse. Some people think that gravityconsists of particles called gravitons,which travel at the speed of light.
Theonly thing we do know is how gravityoperates in different parts of our universe. Without gravity, there would be no spaceand time. There is a legend that says that Galileoonce dropped two objects off the LeaningTower of Pisa to show that the heavier ofthe two objectsdropped faster. If a feather and hammerwere the two objects he used thenobviously the hammer would hit theground first. This is due to air resistance,which is the force air exerts on a movingobject. This force acts in the oppositedirection to that of the object’s motion.
Inthe case of a falling object, air resistancepushes up as gravity pulls down, whichcauses the object to slow down. WhenGalileo’s experiment was repeated on themoon, the hammer and the feather hit theground at the exact same time. This is dueto the fact that the moon has no atmosphere. Therefore, air resistance doesn’t exist onthe moon. Also, the amount of airresistance on an object depends on thespeed, size, shape, and density of theobject.
The larger the surface area of theobject, the greater the amount of airresistance on it. This is why feathers,leaves, and sheets of paper fall moreslowly than pennies, acorns, andcrumpled balls of paper. There is another legend that states thatwhen Newton was lying against a tree inan orchard, he was struck on the head byan apple. He wondered what provided theacceleration for the apple to fall to theground.
Was this a force of the earth onthe apple? If so, then the apple must exerta force on the earth according toNewton’s law of action/reaction forces. Newton applied this theory unto theplanets, which orbit the sun. He found bystudying astronomical data, that the forcethat held the earth in orbit around the sunwas the same force that drew the appletoward the earth. This was theforce of gravity that is given by this scaryformula:F = Gm1*Gm2gravity _______r^2F equals the force in Newtons, G equalsthe gravitational constant which is 6.
67 *10^-11 Nm^2 | kg^2, m1 and m2 equalthe mass of each body in kilograms, andfinally, r equals the distance between the2 bodies in meters. If all of this is confusing, I feel your pain,because it took me a long time to get thisall down!Another concept that is important tounderstand is terminal velocity. Terminalvelocity is the highest velocity that will bereached by a falling object. As an objectfalls through air, air resistance graduallyincreases until it balances the pull ofgravity. According to the law of inertia,when the forces acting on an object arebalanced, the motion of the object will notchange. When this happens, the fallingobject will stop accelerating.
It willcontinue to fall, but at a constant, finalvelocity. Newton’s laws of motion and law ofgravitation can be used to explain theforces, position and motion of all objectsin the universe. A simpleanalogy of how gravity controls themotion of a planet around the Sun can beshown by a mass on the end of a stringbeing spun around in a horizontal plane at constant speed. The ball hasconstant speed but the direction is alwayschanging so according to the definition ofvelocity the object must be undergoing aconstant acceleration. According toNewton’s second law, for a mass to beaccelerating, it must have a resulting forceacting upon it. The question is: Wheredoes this force come?The forces involved can be examined byconsidering what happens when the stringbreaks.
When the string breaks the mass is nolonger constrained to travel in its circularorbit and moved off in the direction asshown. This indicates that there must be aforce holding the mass in its circular orbit. It is directed towards the center of thecircle and is called the centripetal force. The centripetal force has a resultingcentripetal acceleration. The thing is, youcan extend all of these concepts, andapply them onto the objects like the sun,moon, planets, and even entire galaxies.
The gravitational force of the sun, actingon the earth, keeps the earth in its orbit,preventing it from traveling away intointerstellar space. The gravitational forceof the earth, acting on us, holds us to theearth’s surface. The gravitationalattraction between a person and the earth is proportional the person’smass and inversely proportional to thesquare of the planet’s radius (distancefrom the person to the center). Thisnumber for gravitational attraction iscalled your weight.
Every planet has mass and so everyplanet exerts a gravitational force onnearby objects. We say that planets havegravity. However, what we really mean isthat there is a gravitational force ofattraction between the planet and a personstanding on the planet’s surface. Thisforce depends on the visitor’s mass, theplanet’s mass, and the planet’s radius. Accordingly, people have differentweights on different planets. For example, a person on the moonweighs only about 1/6 as much as onearth.
The moon’s radius is 25% earth’sradius and the moon’s mass is 8% ofearth’s mass. So, if a student weighs 150pounds on earth, they would weigh only(1/6) * 150 pounds, which equals 25pounds, on the moon. Gravity does more than just keepingplanets orbiting the sun and causingpeople to have weight, gravity also causestides. In simple terms, the tides are caused by the gravitational attractionbetween the moon and earth’s oceansAND by the motion of earth throughouter space. Einstein predicted gravitational waves. They are best understood in comparisonwith electromagnetic waves, which werepredicted by Maxwell in 1864 anddiscovered by Hertz 22 years later.
Hertz discovers electromagnetic waves in1886. Electromagnetic waves are wavesof electricity. They give us our sense ofvision with which to see the universe. Gravitational waves are waves of gravity. They are vibrations of space itself.
Theytravel through space at the speed of light,but are more like sound than light. Hertz’ discovery set the foundation forthe electronic revolution of the twentiethcentury. Electromagnetic waves not onlyrevolutionized our lives, but also ourknowledge of the universe. Astronomersgradually opened the electromagneticspectrum, first using visible light and thenradio, x-rays and gamma rays. Each newpart of the spectrum provided us withdramatic new insights into the universe.
Einstein predicted gravitational waves in1916. They have not yet been directlydetected on earth, although astronomersJoe Taylor and Russell Hulse received the1993 Nobel Prize for proof of theirexistence, by showing that a star system is losing energy by producinggravitational waves. Gravitational waves are a completely newspectrum. If electromagnetic waves let ussee the universe, gravitational waves willlet us hear the universe. They will provideus with a new sense, the sense of hearing,with which to explore the universe. Gravity is a very complicated subject, butscientists are learning more and moreabout it as time goes on.
Contributionsfrom people such as Newton and Einsteinhelped shape the way we see thingstoday. Without them, no telling what kindof misconceptions we all might believe intoday.