# s4a10 - Solutions Homework 4a10 Physics E-1a Fall 2010 MC 1...

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Physics E-1a Solutions Homework 4a10 Fall 2010 MC 1 the answer is a) Draw a free body diagram for the problem and find the net force on the stone due to the tension and gravity. With Newton's second law we have F = T + mg = ma = m v 2 r ! if T = 3mg then F ! = 4 mg = m v 2 r " r 4 g ( ) = v 2 " v = 2 gr MC 2 The answer is c) Twice as big. For the first case a 1 = v 2 R and for the second case, a 2 = 2 v ( ) 2 2 R = 2 v 2 R MC 3 The answer is d) g/4 from ma = G mM r 2 ! g = G M r 2 . Now if we double r (go to an orbit around the earth at a distance of one earth radius from the surface) then the acceleration or g decreases by a factor of four. g at 2 r ( ) = G M 2 r ( ) 2 = G M 4 r 2 MC 4 The correct answer is b) There is no force that pushes you against the door. The centripetal force (center directed force) is the friction force acting on the tires and causing the car to turn in a circle. Your body wants to continue straight ahead so the door pushes on you. So answer a) is false and b) is true, c) is false, remember the door pushes on you, d) is false, the centripetal force acts on you from the tire friction through the car frame to the seat, e) is false, after all, that’s what we have been doing for the last three weeks. 12.(a) Strategy and Solution The force of static friction between the inside wall of the rotor and the people’s backs keeps them from falling.

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s4a10 - Solutions Homework 4a10 Physics E-1a Fall 2010 MC 1...

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