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Forces
 A force is a push or pull upon an object
resulting from the object's interaction with
another object.
 Whenever there is an interaction between two
objects, there is a force upon each of the
objects.
 When the interaction ceases, the two objects
no longer experience the force.
 Forces only exist as a result of an interaction.
Two Main Forces
 For simplicity sake, all forces
(interactions) between objects can be
placed into two broad categories:
 contact forces, and
 forces resulting from action-at-a-distance
 Contact forces are types of forces in
which the two interacting objects are
physically contacting each other.
Examples of contact forces include
frictional forces, tensional forces, normal
forces, air resistance forces, and applied
forces.
 Action-at-a-distance forces are types of
forces in which the two interacting
objects are not in physical contact with
each other, yet are able to exert a push
or pull despite a physical separation.
 Examples of action-at-a-distance forces
include gravitational forces, electric
forces and magnetic forces.
Unit of Force
 Force is a quantity which is measured using the
standard metric unit known as the Newton.
 One Newton is the amount of force required to give a
1-kg mass an acceleration of 1 ms-2.
 A Newton is abbreviated by a "N." To say "10.0 N"
means 10.0 Newtons of force. Thus, the following unit
equivalency can be stated:
 A force is a vector
quantity.
 Because a force is a
vector which has
magnitude and
direction, it is
common to represent
forces using
diagrams in which a
force is represented
by an arrow.
Balanced
Unbalanced
Determining the Net Force
 An object at rest
tends to stay at
rest and an object
in motion tends to
stay in motion
with the same
speed and in the
same direction
unless acted
upon by an
unbalanced
force.
Free Body Diagrams
 Free-body diagrams
are diagrams used to
show the relative
magnitude and
direction of all forces
acting upon an object
in a given situation.
Construct free-body
diagrams for the various
situations described.
 A book is at rest on a table top. Diagram
the forces acting on the book.
 A girl is suspended motionless from a bar
which hangs from the ceiling by two ropes.
 An egg is free-falling from a nest in a tree.
Neglect air resistance. Diagram the forces
acting on the egg as it is falling.
 A flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity.
Consider air resistance. Diagram the forces
acting on the squirrel.
 A rightward force is applied to a book in order
to move it across a desk with a rightward
acceleration. Consider frictional forces. Neglect
air resistance.
 A rightward force is applied to a book in order
to move it across a desk at constant velocity.
Consider frictional forces. Neglect air
resistance.
 A college student rests a backpack upon his
shoulder. The pack is suspended motionless by
one strap from one shoulder.
 A skydiver is descending with a constant
velocity. Consider air resistance.
 A force is applied to the right to drag a sled
across loosely-packed snow with a rightward
acceleration.
Free-body diagrams for four situations are shown below.
For each situation, determine the net force acting upon
the object.
Now Try This!
 Free-body diagrams for four situations are shown
below. The net force is known for each situation.
However, the magnitudes of a few of the individual
forces are not known.
 Analyze each situation individually and determine the
magnitude of the unknown forces.