coefficient of friction

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