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Newton's Laws of Motion
Article by: Charles J. Goebel, Department of Physics, University of Wisconsin, Madison, Wisconsin.
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Newton's laws of motion are three fundamental principles that form
the basis of classical, or Newtonian, mechanics (Fig. 1). These laws
have proved valid for all mechanical problems not involving speeds
comparable with the speed of light and not involving atomic and
subatomic particles.
Newton's laws of motion are as follows:
1. A body not acted on by a force moves with constant velocity, that is,
with constant speed in a straight line.
2. The acceleration (rate of change of velocity with time) of a body is
proportional to the force acting on it, and inversely proportional
to its mass. For precision, if the body is large and irregular or soft,
the words "of a body" must be replaced by "of the center of mass
of a body."
3. Two bodies exert forces on one another that are equal in
magnitude, but opposite in direction (Fig. 2).
The first law is merely a special case of the second law when the force
is zero.
The second law (Fig. 1) is usually stated as "force equals mass multi-
plied by acceleration," F = m a, or as "force equals the rate of change of
momentum." Although this statement is often regarded as merely defin-
ing the force F on a body given its mass m and acceleration a, the fact
drawn from experience by Isaac Newton is that a nonzero force always
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Key Concepts
• Newton's laws of motion comprise three fundamental principles that form the basis of classical mechanics.
• The first law of motion states that a body not acted on by a force moves with constant velocity.
• The second law of motion states that the acceleration of a body is proportional to the force acting on it,
and inversely proportional to its mass.
• The third law of motion states that two bodies exert forces on one another that are equal in magnitude,
but opposite in direction.
Fig. 1: Newton's laws of motion are three fundamental principles of classical
mechanics. Newton's second law of motion states that the acceleration (a) of a
body is proportional to the force (F) acting on it, and inversely proportional to
its mass (m
1
or m
2
). (Copyright © McGraw-Hill Education)
