p140f04_exam_2_1_sol - Physics 140 Fall 2004: Exam #2...

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Physics 140 Fall 2004: Exam #2 Solutions Please print your name:_____________________________________________ Please list your discussion section number:______________________________ Please list your discussion instructor:___________________________________ Form #1 Instructions 1. Fill in your name above 2. This will be a 1.5 hour, closed book exam. The exam includes 20 questions. 3. You may use a calculator, please do not share calculators 4. You may use two 3”x5” note cards with notes and equations you think may be useful. You can write on both sides of the cards if you like. 5. You will be asked to show your University student ID card when you turn in your exam. ID checked by:___________________________________ Discussion section reminder: Section 004 M/W 8:00-9:00 Don Smith Section 005 M/W 9:00-10:00 Don Smith Section 006 M/W 10:00-11:00 Don Smith Section 007 M/W 9:00-10:00 Roy Clarke Section 008 M/W 10:00-11:00 Roy Clarke Section 009 M/W 11:00-12:00 Roy Clarke Section 010 M/W 12:00-1:00 Don Smith Section 011 M/W 1:00-2:00 Don Smith Section 012 M/W 2:00-3:00 Don Smith Section 013 M/W 10:00-11:00 Carl Akerlof Section 014 M/W 11:00-12:00 Carl Akerlof Section 015 M/W 12:00-1:00 Jay Chapman Section 023 M/W 1:00-2:00 Noemi Mirkin Section 024 M/W 2:00-3:00 Noemi Mirkin
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1: A neutron (m n = 1 AMU) collides head-on with a deuteron (m d = 2 AMU). Predict the neutron recoil velocity if the initial velocity is v i in the positive x-direction and the collision between the two is totally elastic. a) -2/3v i b) *-1/3v i c) 0 d) 1/3v i e) 2/3v i 2: A person enters a building through a revolving door. Initially the door is at rest. The person pushes on the door with a constant tangential force F at a distance D from the axis of the door. If the moment of inertia of the door is I, the time taken for the person to enter through the revolving door is: a) (2 π ID/F) 1/2 b) (2 π F/DI) 1/2 c) *(2 π I/FD) 1/2 d) (2 π FD/I) 1/2 e) (2 π D/FI) 1/2 F D This collision conserves both momentum and energy: mv i = mv 1f + 2mv 2f or v 1f = v i – 2v 2f 1/2mv i 2 = 1/2mv 1f 2 + ½(2m)v 2f 2 mv i 2 = m(v i – 2v 2f ) 2 + (2m)v 2f 2 v i 2 = (v i 2 – 4v i v 2f + 4v 2f 2 ) + 2v 2f 2 4mv i v 2f = 6mv 2f 2 or v 2f = 2/3v i and v 1f = v i – 2v 2f = -1/3v i Angular acceleration α = τ / I = FD/I Angular displacement θ = 1/2 α t 2 or t = (2 θ / α ) 1/2 We want to know how long it takes to go through an angle θ = π , so T = (2 θ / α ) 1/2 = (2 π I/FD) 1/2
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3: The 1996 film Eraser introduces a fictional weapon called a "rail gun", which fires pellets at *extremely* high speeds. According to the dialogue, they move at "nearly the speed of light". In a dramatic scene near the climax of the film, Arnold Schwarzenegger is shown holding one of these guns in each hand, firing streams of high-speed bullets at his enemies. If we assume Arnold has a mass of 150 kg (holding the two guns), and that he fires a single bullet of mass 1 gram at 2.7x10 8 m/s (90% the speed of light), how fast
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p140f04_exam_2_1_sol - Physics 140 Fall 2004: Exam #2...

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