1 answerLast reply by: Professor Selhorst-Jones Thu Sep 6, 2012 4:48 PMPost by Patrick Gomez on August 7, 2012I 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 answersLast reply by: Professor Selhorst-Jones Wed Oct 17, 2012 1:58 PMPost by Nik Googooli on August 30, 201250/m.g=50/98=0.51 not 0.71 1 answerLast reply by: Professor Selhorst-Jones Sun Oct 28, 2012 9:49 PMPost by varsha sharma on October 28, 2012in example 3 shouldn't it bemg-fric.= 0 ( because the object is moving down )(though by doing your way ,the answer will be the same)

### 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

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