PHYSICS 8A-02 (01-28-10) - PHYSICS 8A Professor Joel Fajans...

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PHYSICS 8A Professor Joel Fajans 1/28/10 Lecture 4 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 Today is your last day to decide whether or not you want to take this class because this is an early drop class. If you don’t drop by Friday, you are stuck with this class! LECTURE I will continue our discussion of vectors. The problem I would like to discuss is a physical phenomenon. As Californians, you should all be familiar with riptides. Riptides are phenomena that occur when there is a sharp and strong current that goes outwards. Rather than coming in, there is a current that goes outward. It is dangerous because it can pull you out to sea pretty quickly. If you’re trapped in a riptide, swim sideways. Suppose you are 100m from the shore, and swimming at a velocity of 2m/s towards the shore. There is a riptide against you at a velocity of 1.5m/s. Riptides are a great example of the addition of velocity vectors. We add the velocity vectors as follows: ݒ ௡௘௧ ൌ2ݕ ොെ1 .5ݕ 0.5݉ ݏݕ ݐൌ 100 0.5 ൌ 200ݏ This is a long time to be maintaining this pace. A better strategy is to swim sideways. The saving grace about riptides is they are not very wide, so typically you do not have to swim very far to get out of it. ݒ ௡௘௧ You are swimming sideways, but the riptide is still carrying you out to sea. Does this matter? In the long run, yes, but in terms of time, does this matter? No. All we care about is our velocity sideways. ݐ 10݉ ݏ ൌ5ݏ 100m 2m/s 2m/s 1.5m/s 100m 1.5m/s
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PHYSICS 8A ASUC Lecture Notes Online: Approved by the UC Board of Regents 1/28/10 D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. 2 This is much shorter. But note you are being carried out to sea. ݕ ௜௡௖ 1.5݉ ݏ ·5ݏൌ7 .5݉ You have been carried out to sea 7.5m. You only have to swim at the fast rate of 2m/s for 5s to get out of the riptide. Then you can swim slowly back to shore. But if you could sustain that fast swimming rate, how much time would it take to get back to shore?
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PHYSICS 8A-02 (01-28-10) - PHYSICS 8A Professor Joel Fajans...

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