coefficient of friction
TRANSCRIPT
GOOD AFTERNOON!
• Today we will:
• take notes on friction
• compete the Physics To Go 2.7• compete the Physics To Go 2.7
•Do before the tardy bell:
• get a textbook
• get out your spiral & loose-leaf paper
• get out something to write with
WARM-UP: ANSWER ON
LOOSE-LEAF PAPER!!
•You and a teammate are playing
soccer. Your friend complains that
she can’t “get a grip” on the ground.she can’t “get a grip” on the ground.
•What does she mean?
•What advice would you give your
friend?
•You have 5 minutes
COEF F I C I EN T OF F R I C T ION
THE MU OF THE SHOE
FRICTION
• friction is a force that resists relative
motion between two bodies in
contactcontact
• in other words, friction is a force that
resists the motion of an object
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
FRICTION
•We know that the force of the box
downward on the table was
“balanced” by the upward force of “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.
FRICTION
• The box moved at a constant speed (so it wasn’t accelerating). Therefore, there was another force on the box of equal strength but in the opposite direction to the force you applied!applied!
• What was this force?
• Friction!
• How do we know it was a force of equal strength?
• Read pg 212 – 214 now
FRICTION
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 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 okay?
COEFFICIENT OF SLIDING
FRICTION
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 cause the shoe to slide with constant
speed what is the coefficient of sliding
friction?
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
COEFFICIENT OF SLIDING
FRICTION
•µ is valid only for the pair of surfaces
in contact when the value is
measuredmeasured
• any significant change in either surface
may cause the value of µ to change
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. 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
THE GREEK ALPHABET
• there are not enough letters in the
English alphabet to provide the
number of symbols needed in number of symbols needed in
physics, so scientists use letters from
other alphabets
•µ is a Greek letter – it is pronounced
“mew”
FRICTION
•why was the lab called “Mu of the
Shoe”?
PHYSICS TO GO 2.7
•15 minutes to work independently
•12 minutes to work collaboratively•12 minutes to work collaboratively