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Friction
- Friction changes depending on the two materials involved. Wood on rubber is different than wood on wood is different than wood on ice. (This idea is captured by our coefficient of friction: μ.)
- Friction changes depending on how hard the two materials are pushed together. (This idea is captured by the normal force between the two materials: FN.)
- Friction changes depending whether or not the two materials are already in motion relative to each other-static vs. kinetic. (This idea is captured by having two different coefficients of friction: μs and μk.)
- Friction always opposes motion. Whatever direction the object has (the direction of →v), friction points the opposite way.
- The formula for friction is
Ffric = μ·FN. - Kinetic friction is just a continual force of Ffric = μk ·FN, pointing opposite whatever the current direction of movement is.
- Static friction is a little different. It opposes the force on the object until it is overcome, at which point it switches to kinetic friction. It can cancel out other forces, but it never exceeds them.
Maximum Static Friction = μs ·FN. - As usual, be careful when figuring out where all the forces go. A good free-body diagram goes a long, long way. And be extra careful when figuring out the normal force!
Friction
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
- Intro
- Introduction
- The Normal Force and Friction
- Friction as an Equation
- The Direction of Friction
- A Quick Example
- Static vs. Kinetic
- How to Use Static Friction
- Some Examples of μs and μk
- A Remark on Wheels
- Example 1: Calculating μs and μk
- Example 2: At What Angle Does the Block Begin to Slide?
- Example 3: A Block is Against a Wall, Sliding Down
- Example 4: Two Blocks Sitting Atop Each Other































1 answer
Thu Sep 6, 2012 4:48 PM
Post by Patrick Gomez on August 7, 2012
I love Physics! It's amazing how a person's whole way of viewing the world around them changes as they continue to learn more.
3 answers
Wed Oct 17, 2012 1:58 PM
Post by Nik Googooli on August 30, 2012
50/m.g=
50/98=0.51 not 0.71
1 answer
Sun Oct 28, 2012 9:49 PM
Post by varsha sharma on October 28, 2012
in example 3 shouldn't it be
mg-fric.= 0 ( because the object is moving down )
(though by doing your way ,the answer will be the same)