PHYSICS 8A-02 (02-16-10)

# PHYSICS 8A-02 (02-16-10) - PHYSICS 8A Professor Joel Fajans...

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PHYSICS 8A Professor Joel Fajans 2/16/10 Lecture 9 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 We will start off with a discussion of centripetal forces. I have here a normal chain. What I’d like to do is put this chain on this wheel. I’ve got a wheel that turns quickly. I will use the stick to push the chain off the spinning wheel. Why did this happen? This chain is completely flexible. But when it is spinning quickly, what’s necessary to keep something spinning in a circle? Centripetal force, which pushes every element in the chain inwards. There is no obvious string going towards the center. Where is that force coming from? The chain isn’t entirely straight. It is bent just a little bit. One element is being tugged on the two elements surrounding it, and it is being tugged inwards. You can see that the element is being pulled downwards and outwards on one side by its nearby elements. These provide the centripetal force . Why is it more stable in a circle than some other shape? It is the most stable shape for the object to stay in. Let’s return to our discussion of Newton’s laws . Newton’s second law was that the force is equal to the time derivative of the velocity. This could also be written This explains how hammers work: here is a block of wood, one nail, and a hammer. Why is it more effective to hammer the nail down instead of pushing it down? When I hit this hammer nail, there are several sounds. The initial sound you hear is a rather sharp attack-like sound, like a pair of cymbals crashing together. The net result is that we can estimate the time to slow the hammerhead down is very short, perhaps on the order of a millisecond. So if we take our equation, we can calculate what

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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 (02-16-10) - PHYSICS 8A Professor Joel Fajans...

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