Not quite a year after Galileo's death, a weak child, Isaac Newton, was born in the village of Woolsthrope in England. He was to become a professor of physics, a great scientist, a Member of Parliament and President of the Royal Society. Last but not least, Newton discovered universal gravitation, one of the greatest discoveries which scientific investigation ever yielded to mankind. They say that in 1665 while staying in his native village, he noticed by chance that an apple fell from a tree. That insignificant, as it may seem, fact turned his mind to the problem of gravitation. However, to carry on scientific investigations was not so easy about three hundred years ago and it took him about twenty years to complete his theory of gravitation.

Newton was the first to realize the elliptical path of comets and was the discoverer of the three basic laws of motion which are the foundation of practical mechanics.

Newton's Laws of Motion to be discussed in this article are based upon his own and Galileo's experiments. His laws of motion are to be modified when speed approaches the speed of light. However, for practical mechanical problems, the laws of motion hold good to this day.

Newton's First Law of Motion may be expressed as follows: Any object remains at rest or continues to move at constant speed in a straight line, unless it is acted upon by some force.

The rate of speeding up is commonly termed "acceleration", while the rate of slowing down is termed "decelera­tion". In other words, to "accelerate" is to increase speed and to "decelerate" means to slow down motion. In science, the one term acceleration is often used to designate both types of motion. The acceleration of an automobile is positive when force is supplied by the motor to speed up the car. The acceleration is negative when the retarding force of the brakes slows down the motion of the car. We even speak of acceleration when the motorcar goes round a corner at constant speed because a change in the direction of motion requires a force just as a change of speed does.

Newton's Second Law of Motion expresses the relation between force, mass and acceleration as follows: The acceleration of a body is directly proportional to the force acting and is inversely proportional to the mass of the body.

The Second Law may perhaps be expressed more simply in the form of an equation:

Force=Mass × Acceleration,

or

This is one of the equations to be used when we want to calculate changes in speed.

Newton's Third Law of Motion may be expressed as follows: Forces always act in pairs and the two forces in a pair are equal and opposite.

To every action there is an opposite and equal reaction.

It is to be noted that the action and reaction, as the two forces of a pair are generally called, always act on different bodies and never act on the same body. Also, action and reaction are equal even when a body is accelerated.

As an illustration of the Third Law, suppose that some person is pushing a door in order to open it. Do you know that the door, in its turn, will push back on his hand with an equal and opposite force?

As we know at present Newton's Third Law cannot be applied to force acting at a distance. On the other hand, it holds good for objects at rest and for contact forces.