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What is a force?
 A soccer ball is kicked toward the net, and the
goalie stops the ball just in time by kicking it back
in the opposite direction.
 What caused the ball to start moving, stop moving,
and change direction?
 The answer is force!
 Force is a push or a pull that causes an object to
start moving, stop moving, or change direction.
How do we describe force?
 Force is described by its strength and
direction, much like velocity and
 The strength of force is measured in the SI
Unit called the newton.
Named after Sir Isaac Newton
 The symbol for a newton is (N).
Strength and Direction of force
 The strength and direction of force is represented by
an arrow.
 The direction of the arrow tells you in which
direction the force is acting.
 The length of the arrow tells you how strong the
force is.
The longer the arrow, the greater the force.
Combining Forces
 Most of the time, more than one force acts on an object at
one time.
The combination of all the forces acting on an object is
called the net force.
When forces act in the same direction the forces are added
together. 5N
+ 5N
= 10N
When the forces act in opposite directions, the forces are
subtracted, much like adding a positive and a negative
number. 10N
+ 5N
= 5N
When equal forces act in opposite directions they cancel
each other out – leaving a net force of zero.
Balanced and Unbalanced Forces
 Unbalanced forces occur whenever there is a net
force acting on an object.
 Unbalanced forces ALWAYS cause a change in the
object’s motion.
The object may start moving, stop moving, or change direction.
 Balanced forces occur when equal forces act on an
object in opposite directions.
Each force is balanced by the other.
Balanced forces do not cause an object’s motion to change.
Calculating Net Force
 Practice force problem: (chp.10, p.337)
 You exert a force of 120N on a desk. Your friend exerts a force
of 150N in the same direction. What is the net force you and
your friend put on the desk?
 The net force is 270N
Exit Slip
 How do balanced forces acting on an object affect
the object’s motion?
 How do unbalanced forces acting on an object
affect the object’s motion?
 Balanced forces cancel each other out and do not change
the object’s motion.
 Unbalanced forces cause the object’s motion to change by
either causing it to start moving, stop moving, or change