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Type of Force
(and Symbol)
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Description of Force
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Applied
Force
Fapp
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An
applied force is a force that 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)
Fgrav
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The
force of gravity is the force with 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 that 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:
Fgrav = m * g
where g = 9.8 N/kg (on Earth)
and m = mass (in kg)
(Caution: do not confuse weight with mass.)
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Normal
Force
Fnorm
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The
normal force is the support force exerted upon an object that 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 that are in contact with each other. For
instance, if a person leans against a wall, the wall pushes horizontally on
the person.
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Friction
Force
Ffrict
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The
friction force is the force exerted by a surface as an object moves across it
or makes an effort to move across it. There are at least two types of
friction force - sliding and static friction. Though it is not always the
case, the friction force often opposes the motion of an object. For example,
if a book slides across the surface of a desk, then the desk exerts a
friction force in the opposite direction of its motion. Friction results from
the two surfaces being pressed together closely, causing intermolecular
attractive forces between molecules of different surfaces. As such, friction
depends upon the nature of the two surfaces and upon the degree to which they
are pressed together. The maximum amount of friction force that a surface can
exert upon an object can be calculated using the formula below:
Ffrict = µ • Fnorm
The
friction force is discussed in more detail later
on this page.
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Air
Resistance Force
Fair
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The
air resistance is a special type of frictional force that acts upon objects
as they travel through the air. The force of air resistance is often observed
to oppose the motion of an object. This force will frequently be neglected
due to its negligible magnitude (and due to the fact that it is
mathematically difficult to predict its value). It is most noticeable for
objects that travel at high speeds (e.g., a skydiver or a downhill skier) or
for objects with large surface areas. Air resistance will be discussed in more detail in
Lesson 3.
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Tension
Force
Ftens
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The
tension force is the force that is transmitted through a string, rope, cable
or wire when it is pulled tight by forces acting from opposite ends. The
tension force is directed along the length of the wire and pulls equally on
the objects on the opposite ends of the wire.
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Spring
Force
Fspring
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The
spring force is the force exerted by a compressed or stretched spring upon
any object that is attached to it. An object that compresses or stretches a
spring is always acted upon by a force that restores the object to its rest
or equilibrium position. For most springs (specifically, for those that are
said to obey "Hooke's Law"), the magnitude of the force is directly
proportional to the amount of stretch or compression of the spring.
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