Lect17-Phys172s11-(9.4-9.7-Multiparticle_systems,Exam2results)

Lect17-Phys172s11-(9.4-9.7-Multiparticle_systems,Exam2results)

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Unformatted text preview: Spring 2011 PHYS 172: Modern Mechanics Lecture 17 Multiparticle system, Collisions Read 9.4 9.7 EXAM 2 results Averages: Multiple choice: 45/70 = 64% Handwritten: 14/30 = 47% Total: 59/100= 59% EXAM PROBLEM REVIEW Two cars are accelerated from rest under full throttle. While car 1 is heavier than car 2, the engines in both cars push them with the same constant force along the first 20 m of track. As the cars reach the end of the 20 m track, their momenta and kinetic energies are denoted as p 1 , p 2 , K 1 , K 2 , respectively, with the subscript 1 for the heavier car, and 2 for the lighter car. Which one of the following statements is correct? A) K 1 = K 2, p 1 > p 2 36 % B) K 1 = K 2, p 1 = p 2 20 % C) K 1 < K 2, p 1 < p 2 12 % D) K 1 = K 2, p 1 < p 2 7 % E) K 1 > K 2, p 1 > p 2 18 % Hardest problem (36% right answers) 1) Energy principle: convert work into kinetic energy. K = W = F . d = the same! 2) Momentum principle: net p F t = & & Which car will cover 20 m in longer time? System: car; External force: Earth push on wheels (mg and normal force cancel) Similar to clicker question: slide 12, lecture 9 EXAM PROBLEM REVIEW Energy principle: (nonrelativistic) grav K U const + = The escape speed from a very small asteroid is only 16 m/s. If you throw a rock away from the asteroid at a speed of 20 m/s, what will be its final speed very far away from the asteroid? (Ignore interactions with anything else in the universe). A) 12 m/s 44% B) 4 m/s 23 % C) 2 m/s 8 % D) 8 m/s 8 % E) 3.6 m/s 10 % System: rock+asteroid. Approximation: no interaction with surrounding after you throw the rock Escape speed: grav K U + = 2 2 grav esc U mv = - f i grav K K U + = + Higher speed: NOTE: homework problem! 2 2 2 2 2 2 f i esc mv mv mv =- 2 2 f i esc v v v =- (44% right answers) EXAM PROBLEM REVIEW Air pressure at the surface of a fresh water lake near sea level is about 10 5 N/m 2 ....
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Lect17-Phys172s11-(9.4-9.7-Multiparticle_systems,Exam2results)

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