FORCES AND FRICTION

1
FORCES AND FRICTION
2
Bellringer 6
3
Bellringer 7

• A force is
• a. Anything that is in motion.
• b. Any push or pull on an object.
• c. Anything that reacts with another object.
• Motion is a result of
• a. Any force, balanced or unbalanced.
• b. Only balanced forces.
• c. Only unbalanced forces.
• Friction is a result of
• a. Two objects rubbing against each other.
• b. Tiny hills and valleys on the surface of
  object rubbing together.
• c. Both A and B
• Forces always act in pairs
• a. True
• b. False

4
BELLRINGER
READ THE FIRST PARAGRAPH ON PAGE 11 OF YOUR BOOK.
THINK ABOUT OTHER WAYS YOU HAVE HEARD THE WORD
FORCE USED.
5
WHAT IS A FORCE?
A force is simply a push or a pull.
THERE IS A BIG RELATIONSHIP BETWEEN FORCE AND
MOTION
WITHOUT A FORCE THERE WOULD BE NO MOTION
6
FORCES ALWAYS OCCUR IN PAIRS
Force gives an object the energy to move, stop
moving, or change direction.
7
Examples of forces acting on objects would be
A flag being blown by the force of the wind.
A jet engine propelling an airplane forward.
Iron being pulled toward a magnet.
8
HOW DOES FORCE CAUSE MOTION?
Motion is a result of UNBALANCED
FORCES. UNBALANCED FORCE is the total force on
an object in one direction .
9
HOW DO UNBALANCED FORCES CAUSE MOVEMENT?
REMEMBER Forces always come in pairs.
2 Unbalanced forces acting in the same direction
get added together and cause motion.
10
EXAMPLES OF UNBALANCED FORCES
PUSHING A SHOPPING CART
SKIING DOWNHILL
11
WHAT IS A BALANCED FORCE?
Balanced Forces cause no motion. The paired
forces occur in opposite directions and are of
equal force.
12
EXAMPLES OF BALANCE FORCE
Two dogs of equal strength playing tug of war
Skier on a flat surface
13
LETS PRACTICE
14
WHICH PICTURES SHOW BALANCED FORCE? WHICH
PICTURES SHOW UNBALANCED FORCE?
15
(No Transcript)
16
FRICTION FORCE THAT OPPOSES MOTION Friction -
The force that opposes motion between two
surfaces.
Friction occurs because the surface of any object
is rough. All surfaces are covered with
microscopic hills and valleys. When surfaces
touch, the hills and valleys of one surface stick
to the hills and valleys of the other surface.
This causes friction even if the surfaces look
smooth.
17
Would you rather catch a ball diving in the grass
or on concrete? Why do ice skates only have a
small blade? Why put oil in your car? Why cant
you open a jar with lotion on your hands? All
of these questions deal with friction !!!!!
18
FACTORS THAT AFFECT THE AMOUNT OF FRICTION 1.
Roughness - The more rough the surface more
friction created. 2. Force - The more
force pushing the object the more
friction between valleys. 3. Attraction between
objects(static) - The greater the
attraction the more friction created.
19
Humans have tiny ridges in their hands and feet
which increase the amount of friction. This
allows us to clasp objects and avoid slipping.
Tires are designed to increase friction on the
road. HOW DO THEY DO THIS?
20
The rougher the surface the friction,
____________ because ______________________. If
the force is increased, the hills and valleys of
the surfaces can come closer contact so friction
is _______________. Less massive objects exert
less force on surfaces than more massive objects
so friction is less.
21
TYPES OF FRICTION
1. Sliding Friction
2. Rolling Friction
3. Fluid Friction
22
SLIDING FRICTION Definition - Friction caused by
objects sliding across one another. Examples -
Pushing an object (dresser), sledding, brakes,
chalk
23
ROLLING FRICTION Definition - Friction produced
by objects such as wheels or ball
bearings. Examples - Wheels are placed under
objects to reduce sliding friction. Cars, moving
large objects with dollies,
24
FLUID FRICTION Definition - Force exerted by a
fluid(gas and liquids). Examples - Water, oil,
and air. Sliding friction is usually changed to
fluid friction by adding lubricants. Why is it
on a slick floor?
25
STATIC FRICTION Definition - When a force is
applied, but the object does not move. Examples
- Block and a table. Block will move when the
force is large enough to overcome the friction.
26
WAY FRICTION IS HELPFUL 1. Push against car to
make it move. 2. Brakes to stop objects. 3.
Between pencil and paper. 4. Friction between
floor and feet.
27
WAYS FRICTION CAN BE HARMFUL 1. Friction in the
engine causes temperature to rise. 2. Wind
and water causes erosion. 3. Holes in you
jeans. 4. Hard to move heavy objects.
28
WAYS TO REDUCE FRICTION 1. Lubricant(liquid or
gas) 2. Wheels 3. Ball bearings 4. Make
surfaces that rub against each other
smoother.
29
WAYS TO INCREASE FRICTION 1. Make surface
rougher(sand on ice or batters glove). 2.
Increase the force pushing the surfaces
together.
30
Name the different types of friction. Give
examples of each.
31
Name two ways friction is helpful and harmful.
32
Name two ways to overcome friction.
33
Gravity, Free Fall, and Air Resistance
Gravity - the force that pulls objects toward
one another. Free Fall when only gravity is
acting on an object. This is rare because of
air resistance. All objects free fall at a
rate of 9.8 m/s. A golf ball and basketball
will hit the ground at the same time. Air
Resistance type of fluid friction which
increases with the surface area of the object.