Coefficient of Friction

Coefficient of Friction

• 1.
  GOOD MORNING! • Todaywe 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 INYOUR 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?
• 3.
  THE MU OFTHE SHOE COEFFICIENT OF FRICTION
• 4.
  FRICTION • friction isa 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 ourlab 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 knowthat 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 boxmoved 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?
• 8.
  FRICTION
• 9.
  NORMAL FORCE • NEWVOCABULARY 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?
• 10.
  COEFFICIENT OF SLIDING FRICTION
• 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”
• 16.
  FRICTION • why wasthe lab called “Mu of the Shoe”?