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Forces
TAKEN FROM - CHAPTER 10, SEC. 1
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
acceleration.
 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.
+
=0
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
direction.