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
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?
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
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 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 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?
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?
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?
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 measured • 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.• 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 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”?