Newton's Laws

1. Newton's First Law. When no forces act on a body, that body will either remain at rest or continue to move along a straight line with constant speed.

This is also refered to as the Law of Inertia. Inertia is the reluctance of a body to change its motion. A system in which no forces act is called an intertial frame of reference. This is significant as systems in different inertial reference frames will arrive at the same results in physics.

Equilibrium is achieved when the net force on a body equals zero. Stable equilibrium is a state where a small displacement causes motion back toward equilibrium. While with unstable equilibrium a small displacement results in motion away from equilibrium.

2. Newton's Second Law. The sum of the forces = (mass)(acceleration).

The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass.

3. Newton's Third Law. Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Action = - Reaction.

One Newton is the force required to accelerate a mass of 1 kg by 1 m/s2 in the direction of the force.

PG 73: 1, 2, 3, 4, 6, 7, 9, 10.


The first law really goes back to the first part of the 17th century. It was Galileo who expressed what he called the law of inertia. "A body at rest remains at rest "and a body in motion continues to move "at constant velocity along a straight line unless acted upon by an external force."

Newton's own words in his famous book, Principia. "Every body perseveres in its state of rest" or of uniform motion in a right line "unless it is compelled to change that state by forces impressed upon it."


Walter Lewin