F ma
W
mg
gc
FORCE AND WEIGHT
Application of Newton's Laws
CP-04
Page 2
Rev. 0
Force
Force is defined as a vector quantity that tends to produce an acceleration of a body in the direction
of its application. Changing the body's velocity causes the body to accelerate. Therefore, force can
be mathematically defined as given by Newton's second law of motion (Equation 4-1).
(4-1)
where:
F
=
force on object (Newton or lbf)
m
=
mass of object (Kg or lbm)
a
=
acceleration of object (m/sec or ft/sec )
2
2
Force is characterized by its point of application, its magnitude, and its direction. A force that is
actually distributed over a small area of the body upon which it acts may be considered a concentrated
force if the dimensions of the area involved are small compared with other pertinent dimensions.
Two or more forces may act upon an object without affecting its state of motion. For example, a
book resting upon a table has a downward force acting on it caused by gravity and an upward force
exerted on it from the table top. These two forces cancel and the net force of the book is zero.
This fact can be verified by observing that no change in the state of motion has occurred.
Weight
Weight is a special application of the concept of force. It is defined as the force exerted on an
object by the gravitational field of the earth, or more specifically the pull of the earth on the body.
(4-2)
where:
W
=
weight (lbf)
m
=
mass (lbm) of the object
g
=
the local acceleration of gravity (32.17 ft/sec )
2
g
=
a conversion constant employed to facilitate the use of Newton's second law of
c
motion with the English system of units and is equal to 32.17 ft-lbm/lbf-sec2
Note that g has the same numerical value as the acceleration of gravity at sea level.
c
