Page 65 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 5

Laws of Motion

FIRST LAWOFMOTION

Three centuries ago, Isaac Newton formulated three principles that summarize so much of the behavior of moving
bodies they are called the lawsof motion. These laws are still valid today. According to the first law of motion,
a body at rest remains at rest and a body in motion remains in motion at constant velocity (both magnitude and
direction) if the body does not interact with anything else.
We think of force loosely in terms of pushes or pulls. The first law leads to a more precise definition: A
force is any influence that can change the velocity of a body. Two or more forces can act on a body without
affecting its velocity if the forces cancel one another out. What is needed for a velocity change is a netforce,or
unbalanced force. To accelerate something, a net force must be applied to it. Conversely, every acceleration is
due to the action of a net force.

MASS
The property a body has of resisting any change in its state of rest or of uniform motion is called inertia. The
inertia of a body is related to what we can think of as the amount of matter it contains. A quantitative measure
of inertia is mass: The more mass a body has, the less its acceleration when a given net force acts on it. The SI
unit of mass is the kilogram (kg). A liter of water, which is 1.057 quarts, has a mass of almost exactly 1 kg.

SECOND LAWOFMOTION
According to Newton’s second law of motion, the acceleration a of a body that is acted upon by a force is
proportional to the magnitude of the force and inversely proportional to the body’s mass. The greater the force,
the greater the acceleration, and the greater the mass, the less the acceleration. The direction of the acceleration
is the same as that of the force. Thus a = F/m, which is usually expressed in the equivalent form
F = ma
Net force = (mass)(acceleration)

Net force is sometimes designated F, where (Greek capital letter sigma) means “sum of.” The second law
of motion is the key to understanding the behavior of moving bodies since it links cause (force) and effect
(acceleration) in a definite way.
In the SI system, the unit for force is the newton (N): If a net force of 1 N is applied to a 1-kg mass, the
2
mass will have an acceleration of 1 m/s . The newton is not a basic unit, and in calculations it may have to be
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