**MECHANICS**

The science of *mechanics* deals with the effects of forces in causing or preventing
motion. Statics is the branch of *mechanics* that deals with bodies in equilibrium,
i.e., the forces acting on them cause them to remain at rest or to move with uniform velocity.
Dynamics is the branch of *mechanics* that deals with bodies not in equilibrium, i.e., the
forces acting on them cause them to move with non-uniform velocity. Kinetics is the
branch of dynamics that deals with both the forces acting on bodies and the motions that
they cause. Kinematics is the branch of dynamics that deals only with the motions of bodies
without reference to the forces that cause them.

Definitions of certain terms and quantities as used in *mechanics* follow:

__Force__ may be defined simply as a push or a pull; the push or pull may result from the
force of contact between bodies or from a force, such as magnetism or gravitation, in which
no direct contact takes place.

__Matter__ is any substance that occupies space; gases, liquids, solids, electrons, atoms,
molecules, etc., all fit this definition.

__Inertia__ is the property of matter that causes it to resist any change in its motion or state of
rest.

__Mass__ is a measure of the inertia of a body.

__Work__, in *mechanics*, is the product of force times distance and is expressed by a combination
of units of force and distance, as foot-pounds, inch-pounds, meter-kilograms, etc.

__Power__, in *mechanics*, is the product of force times distance divided by time; it measures
the performance of a given amount of work in a given time. It is the rate of doing work and
as such is expressed in foot-pounds per minute, foot-pounds per second, kilogram-meters
per second, etc.

__Horsepower__ is the unit of power that has been adopted for engineering work. One horsepower
is equal to 33,000 foot-pounds per minute or 550 foot-pounds per second. The kilowatt,
used in electrical work, equals 1.34 horsepower; or 1 horsepower equals 0.746
kilowatt.

__Torque or moment of a force__ is a measure of the tendency of the force to rotate the body
upon which it acts about an axis. The magnitude of the moment due to a force acting in a
plane perpendicular to some axis is obtained by multiplying the force by the perpendicular
distance from the axis to the line of action of the force. (If the axis of rotation is not perpendicular
to the plane of the force, then the components of the force in a plane perpendicular
to the axis of rotation are used to find the resultant moment of the force by finding the
moment of each component and adding these component moments algebraically.)
Moment or torque is commonly expressed in pound-feet, pound-inches, kilogram-meters,
etc.

__Velocity__ is the time-rate of change of distance and is expressed as distance divided by
time, that is, feet per second, miles per hour, centimeters per second, meters per second,
etc.

__Acceleration__ is defined as the time-rate of change of velocity and is expressed as velocity
divided by time or as distance divided by time squared, that is, in feet per second, per
second or feet per second squared; inches per second, per second or inches per second
squared; centimeters per second, per second or centimeters per second squared; etc.