2. Learning Objectives
• I can explain what causes friction and how it affects
the motion of an object.
• I can identify and describe the four types of friction.
• I can give examples of the four types of friction.
4. Forces: Friction
Friction occurs because the surface of any
object is rough. Even surfaces that feel
smooth are covered with microscopic hills
and valleys.
5. Forces: Friction
When two surfaces are in contact, the
hills and valleys of one surface stick to
the hills and valleys of the other surface.
6. Friction Factors
The amount of friction depends on these 2 factors:
1. Roughness of the surfaces
2. The force pushing the surfaces together.
7. Will the tube roll less over a smoother surface or
a rougher surface? Why?
Smooth Surface
Rough Surface
The rougher surface because there are more
hills and valleys and the valleys are deeper.
8. Which has more friction and why?
100 pounds
The 100 pound box because the
force pushing together is higher
so the hills and valleys are in
closer contact.
40 pounds
9. Friction Factors Review
1. Roughness – The rougher the surface, the more
hills and valleys there are and the greater the friction.
2. Force pushing together/weight – Force is
increased, the hills and valleys will be in closer
contact and friction will increase.
10. Amount of Surface Area in fluids
• The amount of surface area affects the friction
between objects in liquids and gases. Example:
Swimming
11. Surface Area in Fluids
• The amount of surface area affects the friction on
a moving object under the following circumstances:
– air resistance (such as the size of a parachute)
– the resistance of an object as it glides through water
(such as a boat).
12. Surface area between Solids
• The amount of surface area in
contact usually does not
affect friction between two
solids.
- Reason larger surface areas
have more contact but the
pressure between the 2 is less.
The increase in friction area is
offset by the reduction in
pressure
14. Sliding Friction
• Sliding friction is a friction
force that opposes the
direction of motion of an
object that occurs when solid
surfaces slide over one
another.
• Examples:
– Writing – pencil point and
paper
– Combing your hair – surface of
comb and strands of hair
15. Fluid Friction
• The force that tries to slow objects down
when they move through a liquid or a gas.
It's also known as "drag", or "air resistance".
• All gases and liquids are fluids.
• Fluid Friction increases as the speed of the
object increases.
• An airplane and a swimmer both experience
fluid friction.
16. Rolling Friction
• Rolling friction is friction
that occurs between
surfaces in motion that
are rolling in which one of
the surfaces is a wheel,
roller, or ball.
• Examples:
– Riding a bike – tires and
ground
– Bowling – ball and lane
17. Static Friction
• Static friction is friction
that occurs when the
surfaces in contact are
at rest (not in motion).
• Examples:
– A book resting on a desk.
– A potted plant sitting on a
sidewalk.
18. Ways to Reduce Friction
• Smooth the surface – less hills and valleys and there are
not as deep.
• Replace sliding with rolling (Use Wheels)
• Add a lubricant like oil, wax, or grease – Fills in the hills
and valleys.
• Less force pushing together – The hills and valleys are
not pushed together as hard so they have less contact.
19. Ways to Increase Friction
• Rough the surface – more hills and valleys and there are
not are deeper.
• Replace rolling with sliding
• More force pushing together – The hills and valleys are
pushed together harder so the hills and valleys are in
closer contact.
21. Glass
Forces: Friction
Turn to a partner and identify surfaces
that may cause more friction.
Sandpaper
Carpet
Gravel
Less Friction?
ICE
Tile
22. Forces: Friction
Friction is both harmful and helpful.
Turn to an elbow partner and identify
some examples of friction being harmful
and friction being helpful.
25. Forces: Friction
Which of the following would NOT help
you move a heavy object across a
concrete floor?
Water, ball bearings, oil, liquid soap,
steel rods, foam rubber
Name three common items you might
use to increase friction.
