Download DIFFERENT FORMS OF FRICTION

DIFFERENT FORMS OF FRICTION
Even though different forms of friction exist, they
have one thing in common: If two objects are in
contact, frictional forces always try to prevent one
object from sliding on the other object.
EXAMPLE: Rub your hand against the table top. You
can feel the friction push against the motion of your
hand. Now rub it the other way. You should feel the
direction of the friction change.
Friction will always slow an object down.
If friction could be removed an object in motion
would continue to move in a straight line with
constant speed.
TYPES OF FRICTION
Have you ever tried pushing something heavy,
like the blocking sled in football or maybe you
helped your parents move something heavy
like a refrigerator? You might have discovered
that nothing happened at first. Then as you
pushed harder and harder, the object
suddenly started to move.
• When you first started to push, friction between the
heavy sled and the ground or the refrigerator and the
floor opposes the force you are exerting and the Net
Force is zero.
• This type of friction is called static friction.
• Static Friction is the type of friction that prevents an
object from moving when a force is applied.
• Static Friction is caused by the attraction between
the atoms on the two surfaces that are in contact.
• Static Friction keeps an object at rest, Sliding
Friction slows down an object that slides.
• EXAMPLE: When you push an object across a
room, you notice the sliding friction between
the bottom of the object and the floor. You
have to keep pushing to overcome the force of
the sliding friction.
• Sliding Friction is due to the microscopic
roughness of the two surfaces.
• EXAMPLE: The brake pads in a car use sliding
friction against the wheels to slow the car.
Bike brakes work the same way.
ASSESSMENT:
What is the difference between Static Friction
and Sliding Friction?
Static Frication prevents
two objects at rest from
moving past one another
and Sliding Friction slows
down two objects moving
against one another.
MORE FRICTION!!!
• Another type of friction is needed to make a
wheel or tire turn, it is called rolling friction.
• Rolling Friction occurs between the ground
and the part of the tire touching the ground.
• Rolling Friction keeps the tire from slipping on
the ground.
EXAMPLE
It’s usually easier to pull a load on a wagon or
cart that has wheels rather than drag it across
the ground, right? This is because Rolling
Friction between the wheels and the ground is
less than Sliding Friction between the load and
the ground.
Force is Required to Change Motion
The first law says there can be no change in
motion without a net force. That includes
slowing down! It takes a net force 9often
friction) to make things slow down.
If forces are truly balanced, a moving object
will keep moving forever with the same speed,
in the same direction.
Inertia
• The first law is often called the law of inertia
because INERTIA is the property of an object
to resist changes in motion.
• Inertia comes from mass.
• Objects with more mass have more inertia.
EXAMPLE
When you first enter the grocery store pushing
the cart it was fairly easy wasn’t it? But once
Mom began putting the groceries in it, it
becomes harder and harder as she keeps
putting in more and more items!
Why?
Because the mass of the cart is increasing!!
Think Back…
Remember…
MASS is the amount of matter an object
contains.
Newton’s Second Law
• Newton’s first law says that a force is needed to
change an object’s motion.
• But what kind of change happens?
Acceleration
• According to Newton’s second law, the amount of
acceleration depends on both the force and the
mass.
Force and Motion Connect Through
Acceleration
• The second law is the connection between
force, mass, and motion.
• The connection occurs through acceleration,
which results in changes in speed and/or
direction.
• ACCELERATION is the change of velocity.
Acceleration is Proportional to Force
• The second law says that acceleration is
proportional to force.
• What does that mean?
• Well, it means that all other things being
equal, if the force doubles, the acceleration
also doubles.
• If the force is reduced by half, the acceleration
is also reduced by half.
EXAMPLE