Coefficient of Friction
- 1. GOOD AFTERNOON! • 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 & loose-leaf paper • get out something to write with
- 2. 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. • What does she mean? • What advice would you give your friend? • You have 5 minutes
- 3. THE MU OF THE SHOE COEFFICIENT OF FRICTION
- 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 box of equal strength but in the opposite direction to the force you applied! • What was this force? • Friction! • How do we know it was a force of equal strength? • Read pg 212 – 214 now
- 8. FRICTION
- 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 okay?
- 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 was the lab called “Mu of the Shoe”?
- 17. PHYSICS TO GO 2.7 • 15 minutes to work independently • 12 minutes to work collaboratively