1. FORCES
•Why does an object move?
•Why does an object change directions?
•Why does an object stop, or start to move?

2. WHAT IS A FORCE?
 Force- push or pull.
 Described by magnitude (strength) and direction
 Measured in Newtons (N)

3. FORCES
 When forces are exerted in the same direction the
two forces are added.
6N+3N=9N

4. FORCES IN OPPOSITE DIRECTIONS
 When forces are exerted in the opposite directions
one force is given a + number the other is given a –
number, and these numbers are added.

5. Balanced and
Unbalanced
Forces

6. UNBALANCED FORCE
 When one force is greater in magnitude than the
other it creates an unbalanced force.
 This is what causes objects to move, stop, and
change directions.

7. + =
Force 1 + Force 2 = Net Force
UNBALANCED FORCE
1 direction

8. NALANCED FORCES
CHANGE THE MOTION
OF AN OBJECT
Unbalanced Forces
change the motion
of an object

9. BALANCED FORCE
 When both forces are equal in magnitude and in
opposite directions it creates a balanced force.
 The net force will then be equal to 0.
 No movement occurs.
I bet this guy hopes the forces stay balanced!!

10. + = 0
2 forces cancel each other
BALANCED FORCES

12. + =
What about this force?
?

14. Describe the balanced and
unbalanced forces in this given
situation

15. THE MOVEMENT OF A CAR
 What are the forces working on this car?
What other two forces are missing?

16. BIG IDEAS
•There are forces that exist in
nature that create movement and
allow objects to become
stationary.
• Without forces, we would have
mayhem.

17. Book sliding to the left

18. SPEED AND
FORCES

19. Name of
Force
Definition Examples
Gravity Gravity is a force that attracts
bodies of matter toward each
other. It is a force that is
everywhere there is matter
The size of the force depends
on two factors: mass and
distance
-Force of attraction
increases as mass
increases
-Force of attraction
increases as distance
decreases
Keeps the planets in orbit
around the sun, moon in orbit
around the earth
Holds us to the earth's
surface. Weight is a measure
of the force of gravity on your
mass
Is responsible for the tides.
Causes acceleration of a
falling object
Responsible for air and water
pressure (weight of air and
water!)
Surface
Tension
Liquid molecules are
attracted to each other (some
more than others!)
Attraction between molecules
causes a “membrane” to form
on the surface of liquids
Allows small insects to walk
on water
Mosquitoes “attach” eggs to
surface of water
Soap, kerosene, can “break”
the surface tension
Blowing bubbles – stretching
surface tension

21. Magnetic An invisible force felt
within the space around
a magnet.
This space, called the
magnetic field, can
either attract (pull) or
repel (push away) other
magnets and some
types of metal.
All magnets have two
poles. These poles are
opposites and are
called the north and
south poles.
Opposite poles attract,
while like poles, repel.
Compasses work
because of Earth’s
magnetic field
Iron and Nickel can
be magnetized - Other
metals cannot
Magnets can be
made with electricity.

22. Electrical A force between two
charged particles
Opposite charges
attract, while like
charges repel.
The size of the force
depends on two factors:
size of charge and
distance of objects
-Force of
attraction/repulsion
increases as the size of
the charge increases
-Force of
attraction/repulsion
increases as distance
decreases
Static Electricity
caused by touching or
rubbing of two
surfaces which causes
buildup of charges
Walking across
carpet – you gain
electrons which are
released when you
touch a doorknob
(attracted because it is
more +)!

23. Turning
Force
causes an object in
uniform circular motion
to move toward the
center of the circular
path.
a car making a turn
the movement of a
roller coaster
When you are riding
in a car and the car
accelerates, your body
tends to move
backward against the
seat. Likewise, if the
car stops suddenly,
your body tends to
move forward, in the
direction of the
dashboard

24. Buoyant
Force
(Upthrust)
Force equal to the weight
of the fluid (liquid or gas!)
that is displaced by the
object.
Determined by an object’s
DENSITY (mass ÷ volume!)
An object will float if
Density of Object < Density
of Fluid
Equal to the weight of the
fluid (liquid or gas!) that is
displaced by the object
Explains why ships don’t
sink
Why we only see about
10% of icebergs (“tip of the
iceburg”)
Friction A force that acts in a
direction opposite to the
motion of a moving object.
Friction will cause a moving
object to slow down and
finally stop
Dependent on two factors
-How hard surfaces are
pushed together
-The materials of which
the surfaces are made
Sliding friction – when
solid objects slide over one
another
Rolling Friction –
produced by wheels, ball
bearings, etc
Fluid (liquid or gas!)
Friction – when an object
moves through a fluid
Lubricants reduce friction