Friction

1
Friction

• Boy, this stuff rubs me the wrong way

2
Friction

• Friction is a contact force when two surfaces
  touch
• Friction is always in the opposite direction of
  the motion

3
How do we get friction?

• Friction force depends on two things
• The surfaces in contact with each other
• For example, sandpaper on wood, pavement on
  rubber, ice on metal
• The mass of the object/surface that is trying to
  move
• We will work with keeping one surface still and
  one surface moving

4
The interaction between two surfaces

• The part of friction that depends on the two
  surfaces interacting is called the coefficient
  of friction
• Abbreviated m
• The larger the m, the more friction force there
  is
• For example, ice and skin has a small m,
  sandpaper and skin has a large m

5
Just to confuse things

• There are two different types of friction
• Static Friction
• The friction force that occurs between two
  objects as one object is trying to start moving
• Kinetic Friction
• The friction force that occurs between two
  objects once one object is moving
• Typically, static friction is higher than kinetic
  friction (easier to keep moving than to start
  moving)

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The Equation

• Mathematically, friction forces look like this
• Ff (m)(FN)

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Example 1

• A person exerts a 36 N horizontal force as he
  pulls a 52 N sled across a cement sidewalk with
  constant speed. What is m between the cement and
  the metal sled runners?

Ff 36 N (because motion is constant speed no
acceleration) Ff (m)(FN) 36 N (m) (52 N) .692
m
8
Example 2

• Suppose the same sled is on packed snow. The m
  now equals .12. If a person weighing 650 N sits
  on the sled, what force is needed to pull the
  sled at constant velocity?

FN 702 N (the weights of the two objects
added) Ff (m)(FN) Ff (.12) (702 N) Ff 84.2 N
9
Friction on slanted surface

• Is friction higher or lower on a slanted surface?
• Well, lets look at the free-body diagram

• Notice what happens to the normal force
• The Fg is acting straight down
• Only a portion of the weight is reflected on the
  FN
• The Ff is smaller!

10
Friction Homework

• Chapter 6
• 100,000,001 101,000,000
• Every 3rd problem, except the palindromes (like
  101,000,101)
• Just Kidding
• 33, 34, 37