1. GOOD MORNING!
• Today we will:
• take notes on friction
• compete the Physics To Go 2.7
• Do before the tardy bell:
• get a textbook
• get out your spiral
• get out something to write with
2. WARM-UP: ANSWER IN YOUR
SPIRAL
• You and a teammate are playing
soccer. Your friend complains that
she can’t “get a grip” on the ground.
• What does she mean?
• What advice would you give your
friend?
4. FRICTION
• friction is a force that resists relative
motion between two bodies in
contact
• in other words, friction is a force that
resists the motion of an object
5. FRICTION
• In our lab you pulled a box at a
constant velocity.
• Newton’s second law tells us that a
constant velocity can only happen
when there is no net force on the box
• Therefore, all of the forces on the shoe
must add up to zero
6. FRICTION
• We know that the force of the box
downward on the table was
“balanced” by the upward force of
the table on the box
• You applied a horizontal force to the
box as you pulled it and measured
this force with your spring scale.
7. FRICTION
• The box moved at a constant speed (so
it wasn’t accelerating). Therefore, there
was another force on the shoe of equal
strength but in the opposite direction to
the force you applied!
• What was this force?
• How do we know it was a force of equal
strength?
9. NORMAL FORCE
• NEW VOCABULARY ALERT!
• The force that is directly perpendicular to the
surface is called the “Normal Force”
• The normal force was equal in strength and
in the opposite direction to the box’s
weight.
• You didn’t measure the normal force, you
measured the weight of the box.
• Why is this equal to the weight of the box?
11. COEFFICIENT OF SLIDING
FRICTION
• Sample Problem
• A shoe has a weight of 5.0 N. If 1.5 N of
applied horizontal force is required to
cause the shoe to slide with constant
speed what is the coefficient of sliding
friction?
12. COEFFICIENT OF SLIDING
FRICTION
• μ does not have any units because it
is a force divided by a force
• μ usually is expressed in decimal form
- for instance:
• 0.85 for rubber on dry concrete
• 0.60 for rubber on wet concrete
13. COEFFICIENT OF SLIDING
FRICTION
• μ is valid only for the pair of surfaces
in contact when the value is
measured
• any significant change in either surface
may cause the value of μ to change
14. COEFFICIENT OF SLIDING
FRICTION
• the situation in this section was
chosen deliberately so that the
perpendicular force (normal force)
was exactly equal to the weight.
• things get more complicated if the
box (or any object) is on a tilted
surface OR the pulling force is angled
upward or downward
15. THE GREEK ALPHABET
• there are not enough letters in the
English alphabet to provide the
number of symbols needed in
physics, so scientists use letters from
other alphabets
• μ is a Greek letter – it is pronounced
“mew”
