The Laws of Motion
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
Applications of Newton’s
Forces of Friction
Classical mechanics describes the relationship between the motion of objects found in our
everyday world and the forces acting on them. As long as the system under study doesn’t in-
volve objects comparable in size to an atom or traveling close to the speed of light, classical
mechanics provides an excellent description of nature.
This chapter introduces Newton’s three laws of motion and his law of gravity. The three
laws are simple and sensible. The Frst law states that a force must be applied to an object in
order to change its velocity. Changing an object’s velocity means accelerating it, which implies
a relationship between force and acceleration. This relationship, the second law, states that
the net force on an object equals the object’s mass times its acceleration. ±inally, the third law
says that whenever we push on something, it pushes back with equal force in the opposite
direction. These are the three laws in a nutshell.
Newton’s three laws, together with his invention of calculus, opened avenues of inquiry
and discovery that are used routinely today in virtually all areas of mathematics, science, engi-
neering, and technology. Newton’s theory of universal gravitation had a similar impact, start-
ing a revolution in celestial mechanics and astronomy that continues to this day. With the ad-
vent of this theory, the orbits of all the planets could be calculated to high precision and the
tides understood. The theory even led to the prediction of “dark stars,” now called black
holes, over two centuries before any evidence for their existence was observed.
three laws of motion, together with his law of gravitation, are considered among the greatest
achievements of the human mind.
is commonly imagined as a push or a pull on some object, perhaps rapidly,
as when we hit a tennis ball with a racket. (See Figure 4.1.) We can hit the ball at
different speeds and direct it into different parts of the opponent’s court. This
Forces exerted by Earth, wind, and
water, properly channeled by the
strength and skill of these windsurfers,
combine to create a non-zero net
force on their surfboards, driving
them forward through the waves.
In 1783, John Michell combined Newton’s theory of light and theory of gravitation, predicting the existence of “dark
stars” from which light itself couldn’t escape.
Tennis champion Andy
Roddick strikes the ball with his
racket, applying a force and directing
the ball into the open part of the
© Lorenzo Ciniglio/Corbis