Friction
Intro to Friction
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Which Experiences More Friction?
Left or right. Which experiences the most friction? Why?
Friction
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Friction Depends Only On:
1. Texture of Each Surface
What do plows have on their wheels to increase friction?
2. Force Holding Objects Together
What do race cars have mounted on their rear hood to increase their friction?
Why do some people put barbells in their car trunk on a snowy day?
Two Kinds of Friction
(fs) 1) Static Friction — force of friction that must be overcome to move an object at rest.
(fk) 2) Kinetic Friction — force of friction that opposes a moving object.
Kinetic friction is weaker than static friction
Friction Equations
fs = force of static friction
fk = force of kinetic friction
Fn = normal force (supporting force)
us = coefficient of static friction — “the roughness number”
uk = coefficient of kinetic friction — “the roughness number”
1. Relates to texture of surfaces in contact
2. Calculated in lab
What do skiers do to their skis to lower the coefficient of friction between their skis and the snow?
What is the direction of the friction at P?
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. What is friction?
2. According to this lesson, friction depends only on…
3. How do static friction and kinetic friction compare for the same two surfaces?
4. A box is pressed against a floor with a normal force of 50 N. The coefficient of kinetic friction is 0.30. What is the kinetic friction force?
5. Why do some drivers put heavy weights in the car trunk on a snowy day?
The Bowler’s Shoes
Bowling shoes have one sole made of smooth leather (for sliding on the approach) and the other made of rubber (for braking). Engineers design soles by choosing materials with the right amount of friction.
According to the friction model used in this course, which two factors determine the force of friction between a shoe and the floor?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. A 600.-N bowler slides toward the pins (to the right) on the leather sole, with all his weight on it. The coefficient of kinetic friction is 0.20. Choose the direction of the friction force on the shoe, and determine its magnitude.
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C2. A bowler weighs 700 N and puts all his weight on the sliding leather sole. If μk = 0.20, what is the friction force?
C3. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response.
C4. The bowler (weight 700 N) brakes with the rubber sole, which has a coefficient of kinetic friction of 0.70 on the approach. Substitute numerical values into a mathematical representation that can be used to determine the friction force on the rubber sole. Then calculate the force.
C5. Why is the braking shoe made of rubber rather than leather?
C6. A bowling alley applies a polish to the approach that lowers the coefficient of friction for all shoes. Identify one benefit and one drawback of this change.
