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My big idea is to introduce my class to the concept of friction. There are three things I'd like to point out about it, and I will in this presentation.
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An Introduction to Friction

Published on Nov 24, 2015

An introduction to sliding friction. Friction always acts against motion, has static and kinetic forms, depends upon the materials in contact, and is sometimes desirable.

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1.

An Introduction to Friction

The force against motion
My big idea is to introduce my class to the concept of friction. There are three things I'd like to point out about it, and I will in this presentation.
2.

Friction is a contact force between surfaces

Uneven surfaces rub against each other, and resist motion
Looks pretty rough, doesn't it? That's a really blown-up look at what's called surface roughness. Interaction between these tiny peaks and valleys gives us sliding friction.
3.

Friction always acts against motion

Force at left must be more than friction force to move object
The block isn't going to move until the force from the left is larger than the friction force that resists motion.
4.

Gravity pulls a block down a plane

If the weight is more than friction! 
The weight of the block is in the red vector that points down. There's also a normal force that depends on the weight of the block and the angle of the plane. The force of friction that resists motion is a product of the coefficient of friction, mu, and this normal force.
5.

The coefficient of friction is the Greek mu 0

The coefficient of friction is the Greek mu, or is sometimes written as just a u.
6.

Static friction must be overcome for something to move, but once it's moving, kinetic friction is smaller.

Once static friction is overcome, and the block slides down the plane, there is a smaller friction force that is still working on the sliding block. That's called kinetic friction.
7.

Kinetic friction coefficient is smaller than that of static friction.

The weight of the block still points down, and either static or kinetic friction gives a force that points up the plane, against motion. Both static and kinetic friction use coefficients of friction, and the force in either one is still the simple product.
8.

The coefficient of friction varies by material.

Various materials have different coefficients of static and kinetic friction. Rubber on concrete is very high, steel on steel is high, copper on steel is lower, and lubricated metal on metal coefficients are the lowest of all shown here.
9.

Friction is sometimes desirable, and sometimes not.

In a work without friction, nothing could move or hold together, so sometimes friction is desirable. When it works against desired motion, though, it is undesirable.
10.

Friction...

  • Always resists motion
  • Has static and kinetic forms
  • Depends upon the materials in contact.

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