# Isaac Newton (1642 - 1727)

Isaac Newton was born 11 months after Galileo died. Among his achievements are:

• His work entitled Principia formulated the laws of motion that are the basis of mechanics. It is probably the greatest scientific work ever written. He therefore explained why the planets follow Kepler's laws.

• He showed that gravity not only makes an apple fall to the ground but also governs the motions of planets and their satellites, and should apply to any bodies (for example, binary stars).

•  He expressed the universal law of gravitation in mathematical form, showing that the force of gravity falls off inversely with the square of the distance between two bodies.

• He further showed that the law of gravitation could explain both tides on the earth and the precession of the equinoxes.

Newton formulated three important laws:

• An object stays at rest if it is at rest or stays in uniform motion in a straight line unless it is acted upon by some force.

• When acted upon by a force, an object changes its motion in an amount proportional to the force acting upon it, and according to the direction of the force.

• When one object exerts a force upon a second object, the second object exerts an equal and opposite force back on the first.