PHYSICS 8A-02 (01-26-10) - PHYSICS 8A Professor Joel Fajans...

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PHYSICS 8A Professor Joel Fajans 1/26/10 Lecture 3 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 Recall the bar trick demo we did in our last class, where I dropped a dollar bill and challenged a member of the class to catch it. We now go through the math explaining the phenomena. ܮൌ 1 2 ܽݐ 1 2 ݃ݐ ݐൌ ඨ ݃ ൌ 0.15ݏ It turns out it takes around 0.2 seconds for the nerve impulses to flow down your arm. Tim couldn’t respond faster than that, but it only took 0.15 seconds for the dollar bill to slip through his fingers. I’d like to talk next about why bike helmets work. Bike helmets are very different from other sorts of helmets. They don’t resemble military helmets at all. The more holes bike helmets have, the more expensive they are. Military helmets obviously don’t have holes. What is the functional difference? The point of a military helmet is that it has a hard shell to stop shrapnel. Typically, people are not shooting at you on a bicycle. The point of a bike helmet is to protect you when you crash. The way it works has to do with the thickness of a helmet. We abandon helmets for a minute to do a demonstration. We have two volunteers hold up a sheet and we will have another volunteer throw an egg at the sheet. No matter how hard the student throws the egg, it does not break. The reason is that the egg is brought to a slow stop and decelerates relatively slowly over a long distance.
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PHYSICS 8A-02 (01-26-10) - PHYSICS 8A Professor Joel Fajans...

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