Type of Force
(and Symbol)
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Description of Force
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Applied Force
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An applied force is a force which is applied to an
object by a person or another object. If a person is pushing
a desk across the room, then there is an applied force
acting upon the object. The applied force is the force
exerted on the desk by the person.
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Gravity Force
(also known as Weight)
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The force of gravity is the force at which the earth,
moon, or other massively large object attracts another
object towards itself. By definition, this is the weight of
the object. All objects upon earth experience a force of
gravity which is directed "downward" towards the center of
the earth. The force of gravity on earth is always equal to
the weight of the object as found by the equation:
 and m
= mass (in kg)
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Normal Force
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The normal force is the support force exerted upon an
object which is in contact with another stable object. For
example, if a book is resting upon a surface, then the
surface is exerting an upward force upon the book in order
to support the weight of the book. On occasions, a normal
force is exerted horizontally between two objects which are
in contact with each other.
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Frictional Force
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The frictional force is the force exerted by a
surface as an object moves across it. The frictional force
opposes the motion of the object. For example, if a book
moves across the surface of a desk, then the desk exerts a
frictional force in the opposite direction of its motion.
The frictional force can be calculated using the
equation:
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Air Resistance
Force
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The air resistance is a special type of frictional
force which acts upon objects as they travel through the
air. Like all frictional forces, the force of air resistance
always opposes the motion of the object. This force will
frequently be neglected due to its negligible magnitude. It
is most noticeable for objects which travel at high speeds
(e.g., a skydiver or a downhill skier) or for objects with
large surface areas.
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Tensional
Force
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The tension is the force which is transmitted through
a string, rope, or wire when it is pulled tight by forces
acting from each end. The tensional force is directed along
the wire and pulls equally on the objects on either end of
the wire.
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Spring Force
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The spring force is the force exerted by a compressed
or stretched spring upon any object which is attached to it.
An object which compresses or stretches a spring is always
acted upon by a force which restores the object to its rest
or equilibrium position. For most springs (specifically, for
those which are said to obey "Hooke's Law"), the magnitude
of the force is directly proportional to the amount of
stretch or compression.
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