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What is Newton's Laws of Motion?

Updated on December 2, 2016

Even today, many people are misled into believing that force is always necessary to make bodies move. Everyday experience indicates that when bodies are not being pushed or pulled they stay still. It therefore seems natural to assume that when bodies are stationary there are no forces acting on them. Yet it is just this misunderstanding that prevents many people from obtaining a clear grasp of the laws of motion. Galileo was the first to realize that when bodies have no forces acting on them they can, once set in motion, continue to move without being pushed or pulled in any way.

In the year in which Galileo died, one of the world's most brilliant scientists was born- Isaac Newton (1642-1727). Not only did Newton believe in an experimental method of approach, but he also had the ability to express his ideas mathematically, and this combination of theory and experiment made it possible for him to see further and deeper into the secrets of nature than man had ever done before.

He expressed Galileo's ideas in more precise laws and these formed part of his famous book Principia (1687), a scientific masterpiece, written entirely in Latin. His laws, expressed in modern language, are as follows:

1. A body will continue in its state of rest, or uniform motion (constant speed) in a straight line, unless acted upon by an external force.

2. The force applied to a body causes it to accelerate in the direction in which the force is applied and the acceleration is directly in proportion to the force applied.

3. To every action there is always an opposing reaction, and action and reaction are always equal and opposite.

It is perhaps only since man has accepted space travel as a reality that he has been able to appreciate the full meaning of these laws. Only in space are bodies free to move without being influenced by the resistance of the air and frictional forces, and it is under these conditions that Newton's laws can clearly be seen in operation.

Once a spacecraft has traveled far enough away from a planet's gravitational field to make the forces on it small, it continues to move through space at a constant speed with its rocket motors cut off, as the first law predicts. The thrust of the rocket motors, when they are switched on, will change the speed, that is either accelerate the spaceship or de-accelerate it, depending on which direction the thrust is applied. This is in agreement with the second law of motion.

The third law is best illustrated by coming down to earth. On the earth's surface bodies can be moved by towing them with a rope. The rope transmits a force or pull on the body, but according to the third law, the body also exerts an equal and opposite pull on the rope, caused by frictional forces, air resistance and gravitational pull.

One other factor is involved which influences the force required to move a body, that is its mass, or the amount of matter that has to be moved. The larger the mass, the larger the force required. All units of force are based on a relationship between mass and acceleration and the unit now recommended for use in most branches of science and engineering is called a 'newton'. One newton is the force required to move a mass of one kilogram with an acceleration of one meter per second per second.

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