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p140f04_exam_2_1_sol

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

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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
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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