PHYSICS 8A-02 (02-9-10)

# PHYSICS 8A-02 - PHYSICS 8A Professor Joel Fajans Lecture 7 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley Do

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PHYSICS 8A Professor Joel Fajans 2/9/10 Lecture 7 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley. Do not share, copy or illegally distribute (electronically or otherwise) these notes. Our student-run program depends on your individual subscription for its continued existence. These notes are copyrighted by the University of California and are for your personal use only. D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. LECTURE Today we will discuss friction. This is a topic that students traditionally have a lot of trouble with. Leonardo da Vinci was among the first to study friction. If you look through his notebooks, you’ll see sketches of instances of friction, such as weights sliding down boards. We will examine this same example in class today. I have here a device which has a force sensor. We’d like to use this to study frictional forces. For instance, if I put this block of wood down and hook it to the force sensor, I’d like to figure out how much force I must use before the block starts moving. From this demonstration we learn that static friction can be represented as That is, the amount static friction is proportional to the normal force. You can see that if I increase the normal force by adding another weight to the block, the force necessary to move the block increases. So the amount of frictional force depends on the weight of the object. Heavier things are harder to push. This shouldn’t come as a huge surprise. But this isn’t what the formula says. The formula uses the normal force. The reason can be illustrated in the following example: Suppose you have an incline plane. What if the incline is straight up and down? There will not be any frictional force. Has the weight changed? No, but the normal force has. The normal force is zero in this case. The coefficient is different for different materials. It ranges from 0.01 in synovial joints in humans, to 0.04 for Teflon on steel, to 0.8 for rubber on concrete, and to 0.4 for glass on glass. Glass on glass is actually not very slippery at all.

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## This note was uploaded on 04/29/2010 for the course PHYSICS 83840p3 taught by Professor Fajans during the Spring '10 term at University of California, Berkeley.

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PHYSICS 8A-02 - PHYSICS 8A Professor Joel Fajans Lecture 7 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley Do

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