The idea of gravity supposedly came to Isaac Newton when he saw an apple fall from a tree. Whether this is true or not, Newton certainly perceived that objects are attracted to the ground by some accelerating force. This observation led Newton to argue that all bodies attract each other through the force of gravity and that the strength of that force drops off with distance squared.
Laws of Motion
The inspiration for Newton’s law of gravitation lay first in his three laws of motion which linked forces, mass and acceleration.
Newton considered the motion of a cannonball. Once fired from the cannon, the ball travels a certain distance before falling to the ground. What would happen if the ball was fired more quickly? Newton proposed it would travel further. He further theorised that if the ball was fired so fast that it travelled far enough in a straight line that the Earth curved away beneath it, the ball would be pulled towards Earth but would then follow a circular orbit.
Without the centrally directed force tying the cannonball to Earth, it would fly off into space. Newton took this further, suggesting that the Moon also hanging in the sky because it is held by the invisible pull of gravity. Without this pull, the Moon too would fly off into space.
Inverse Square Law
Newton tried to quantify his ideas and showed that gravity follows an inverse square law – the strength of gravity decreases by the square of the distance from a body.
Newton’s inverse square law of gravity explained in one equation the orbits of all the planets as described in the three laws of Johannes Kepler. Newton’s law predicted that the planets travelled quicker near the Sun as they followed their elliptical orbits. He suggested that a planet feels a stronger gravitational force from the Sun when it travels close to it, which makes it speed up. As its speed increases the planet moves away from the Sun again, gradually slowing back down.
Universal Law
Newton went on to propose that his theory of gravity applied to everything in the universe. Since every body exerts a gravitational force in proportion to its mass and that force falls off as the inverse square of distance from it, any two bodies attract each other. However, since gravity is a weak force, it is only possible to really observe it acting on very massive bodies such as the Sun and the planets.
Newton’s law of gravitation still gives a basic description of the motion of bodies today but it has been built on by other scientific principles, notably by Einstein and his theory of general relativity. However, Newtonian gravity still works well for most objects and for the behaviour of planets, comets and asteroids in the solar system that are spread over large distances from the Sun where gravity is relatively weak.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
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