PHYSICS 8A-08 (03-02-10)

PHYSICS 8A-08 (03-02-10) - PHYSICS 8A Professor Joel Fajans...

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Professor Joel Fajans 3/2/10 Lecture 13 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. ANNOUNCEMENTS The exam will be held Thursday. I have gotten a few questions regarding the exam. There will not be any questions on the exam involving numbers, so everything will be left in algebraic symbols. There will be four problems on the exam. They will be worth the same number of points, but will not be equal in terms of difficulty. I am hoping that the class average is somewhere between 50-70%. I don’t think an exam with an average of 90% is a good thing because one stupid mistake results in a much poorer grade than deserved. So don’t panic if you can’t do every single problem. This exam should be of similar difficulty as the ones I have posted on bSpace. For this exam, you can have one side of one sheet of 8.5 by 11” paper. For the next exam, you can have two sides. In addition to that, you don’t need to memorize the formulas. LECTURE Today, I will begin talking about power . Power is the rate of change of energy. Typically, if you’re doing work on something, power is the time derivative of the energy of this thing. The power you are putting out to accomplish something is equal to the time derivative of the work you are putting out. We’re at about 100 m off sea level. The road behind us which goes to the top of the hill has a peak of 500 m. So it’s about a 400 m climb to the top of the hill. I’d like to calculate how much energy it’d take to climb to the top of the hill. Suppose the bike and I weigh 100 kg. The distance climbed is 400 m. So the total amount of energy it takes me to climb to the top of Grizzly Peak is Is that actually a lot of energy? What’s the conversion to calories? There are about 4 Joules in 1 calorie. There are 1000 calories in a “food calorie.” So this only corresponds to about 100 “food calories.” Is that a lot? That’s about one slice of bread. This is why exercise is futile! Actually, the calculation isn’t quite that grim. You’re not very efficient. Only about 20% of the calories you ingest can be used by your body to do something besides produce heat. So to get 400,000 Joules of useful work out of your body takes about 500 calories. Let’s try and figure out what the power is. How much power can a typical cyclist put out? Think in terms of light bulbs. Back in the day, I could climb Grizzly Peak in somewhat less than half an hour. The power is going to be equal to the total amount
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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 Berkeley.

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PHYSICS 8A-08 (03-02-10) - PHYSICS 8A Professor Joel Fajans...

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