Hw03soln - Phys ic s 1 4 4 Ho w Thing s Wo r k Ho m e wo r k Se t 3 Due Mo nda y 3 1 0 9 In s t r u c t io n s You ar e r esponsi bl e for your own

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Phys ic s 1 4 4 Ho w T hing s Wo r k Ho m e wo r k Se t # 3 Due Monday 3 / 1 / 0 9 In s t r u c t io n s You ar e r esponsi bl e for your own homewor k sol uti ons. Consi der these pr obl ems as pr acti ce for the exam. Consult me or other students when you have questions, but try to work the problems by your self as much as you can. Collabor ation is allowed, but you must in d ic at e y o u r c o llab o r at o r s , both in the class and outside of it. For every problem, show all your work . Simply writing down a final answer without showing your reasoning and calculations is unacceptable . Draw diag rams and s ke t c he s for problems, where useful. For any essay type questions, make sure to use complete sentences. Problem 1 : Which do you think is more important, conse rvation of e ne rgy or conservation of momentum? Why do you think that conservation laws are so important to physicists? Obviously, this question has no right or wrong answer (well, “Who cares?” is pro bably t he wro ng ans we r) – I ’m interested in your personal opinion, and why you have this opinion. A good answer will discuss the reasons and feelings which prompt you to answer the way you do. A v a r i e t y of r e s p on s e s we r e p os s ible her e. It looked like most people “voted” for conser vation of ener gy over conser vation of momentum. An i m p or t a n t di s t i n c t i on i s t h a t e n e r gy a n d m om e n t u m a r e NOT a t a l l the same thing. So conservation of momentum is not just a d ifferent form of conser vation of ener gy. Addi t i on a l l y , b ot h c a n b e u s e d t o m a k e p r e di c t i on s . Problem 2 : a) Chapter 2, Exercise 22 (p. 79): A horse does work on a cart it’s pulling along a straight, level road at a constant speed. The horse is transferring energy to the cart, so why doesn’t the cart go faster and faster? Where is the energy going? Cl ear l y, f r i c t i on i s i mpor t ant . S o t he f r i c t i on f or c e ac t s oppos i t e t he di r ec t i on of mot i on, and t he forward force of the horse and backward force of friction must add up to a net force of zero so that the velocity r emains constant. Fur ther , the fr iction does negative wor k on the system, tr ansfor ming the ener gy tr ansfer fr om the hor se into the car t into ther mal ener gy ( mostly, along with other kinds of ener gy: sound, defor mation, etc.) instead of kinetic energy. b) Chapter 2, Exercise 40 (p. 79):
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This note was uploaded on 03/27/2011 for the course PHYS 144 taught by Professor ? during the Spring '06 term at Bucknell.

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Hw03soln - Phys ic s 1 4 4 Ho w Thing s Wo r k Ho m e wo r k Se t 3 Due Mo nda y 3 1 0 9 In s t r u c t io n s You ar e r esponsi bl e for your own

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