Friction is a force that resists motion between objects in contact with each other. It causes a moving object to slow down and stop. The size of the frictional force depends on how hard the surfaces are pressed together and the types of materials. Sliding friction occurs when surfaces slide over each other, while rolling friction involves wheels or bearings. Lubricants can reduce frictional forces.

1. 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!)

2. Name of
Force
Definition Examples
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
http://www.youtube.com/watch?v=45yabrnryXk&feature=fvw

3. 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.

4. 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 +)!

5. 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

6. 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”)

7. 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

8. Friction is a force
that resists motion.
It involves objects that
are in contact with each
other.