What Newton came up with.
Over three centuries ago, Isaac Newton invented a new kind of mathematics called calculus so that he could model motion in the natural world using mathematics. Calculus is about measuring change and so calculus became a vital tool in describing the motions of simple objects.
Newton was able to make a mathematical model that encompassed both objects falling because of gravity on Earth, and the motion of heavenly bodies in the skies.
Newton decided that the force of gravity on Earth was the same force that organized the motions of the moon around the Earth and the Earth and all the planets around the sun. He invented a formalism and developed mathematical formulas for calculating the size of the gravitational force both on Earth and in outer space.
One of the important formulas in Newton's model is his law for calculating the force of gravity between two objects 1 and 2 with mass m1 and m2, which are separated by a distance R:
F12 = F21 = G m1 m2 / R2
The constant G is a number that occurs in Nature, like the speed of light c. The constant G is called Newton's gravitational constant.
Newton's law of gravity winds up describing the observed motions of the planets extremely well. Another thing it models quite well is the way the gravitational force felt on the surface of a planet depends on the size and mass of the planet. For example, comparing the gravitational force at the surface of the Earth vs. the moon, we get
Fmoon/Fearth = (Mmoon/Mearth) (Rearth2/Rmoon2)
which is about 1/6, and the astronauts who walked on the moon felt it, too. You can see how much lighter lunar gravity is if you watch films of astronauts moonwalking.
This was an enormous thing Newton did - to invent a new kind of math to build a model that described in the same formula the observed motion of both falling objects on Earth and the planets in the heavens.
BUT unfortunately, Newtonian gravity falls apart when we try to combine it with what we've just learned about Special Relativity.
Gravitation and Gravity
The terms gravity and gravitation are often used to explain the same thing, but there is a definite difference between the two.
Gravitation is the attractive force existing between any two objects that have mass. The force of gravitation pulls objects together.
Gravity is the gravitational force that occurs between the earth and other bodies. Gravity is the force acting to pull objects toward the earth.
Since gravitational force is happening to all matter (objects) in the universe, from the largest galaxies down to the smallest atoms, it is often called universal gravitation. Sir Isaac Newton was the first to fully recognize that the force holding any object to the earth is the same as the force holding the moon, the planets, and other heavenly bodies in their orbits. According to Newton's law of universal gravitation any two masses in the universe attract each other with a gravitational pull. The size of this force is given by:
F is the force between two masses (in Newtons)
m and M are the two masses (in kilograms)
R is the distance between the center of these masses (in metres)
G is the universal constant of gravitation (6.7 x 10-11 Nm2/kg2)
Gravitation is actually a very weak force. The pull is too weak to be felt between two people and it is not even strong enough to pull together two lumps of lead placed right next to each other. It is only when one of the masses is the size of a planet that we can feel the force of gravity.
The huge gravitational force of our nearest star, the Sun, holds together the nine planets of our Solar System. The planets move through space at speeds that just balance the Sun’s gravitational pull, so they are locked into a permanent circle (orbit) around the Sun. Moons orbit planets, and satellites and spacecraft orbit the Earth, in the same way. Satellites are not defying gravity in circling endlessly around the Earth, it is just that they are moving so fast around the Earth that gravity never brings them any closer.
Gravity indicates gravitational force that occurs between the earth and other bodies. Gravity is the force acting to pull objects toward the earth.
Gravity is the force that holds us on the ground and causes objects to fall back to the ground after being thrown in the air.
The force holding objects to the earth's surface depends not only on the Earth's gravitational field but also on other factors, such as the Earth's rotation.
The Earth’s gravitational pull extends out into space in all directions. The further you move away from the center of the Earth the weaker the force becomes.
The measure of the force of gravity on an object is the weight of that object. Weight is measured in newtons (N). The weight of an object changes depending on its location in the universe.
Climb the ladder to go back to the top
Open the door to go home