examF_2004s_sol

# examF_2004s_sol - PHYSICS 221 Spring 2004 Final Exam May 5...

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Physics 221 2004 S Final Exam Solutions Page 1 of 23 PHYSICS 221 Spring 2004 Final Exam: May 5 2004 12:00pm—2:00pm Solutions Part A: Questions 76-95 (20 questions): [Based on Lectures 1-41] [76] Vectors A G and B G start at a corner of a cube of edge length 1. Vector A G lies along the edge of the cube. Vector B G goes to the opposite corner of the cube. What is the value of B A G G ? (A) 0 (B) 1 (C) 2 (D) 3 (E) 2 Take the origin at the base of the two vectors and define the x axis in the direction of the a vector using the co-ordinate system shown. Thus k j i B i A ˆ ˆ ˆ , ˆ + + = = G G . Calculating the dot product: 1 ) ˆ ˆ ˆ ( ) ˆ ( = + + = k j i i B A G G [77] A rock is thrown upwards from a bridge 20.0m above level ground. The initial upwards velocity of the rock is 10.0m/s. How fast is the rock moving when it hits the ground? Neglect air resistance: (A) 17.2 m/s (B) 19.8 m/s (C) 22.2 m/s (D) 25.0 m/s (E) 27.2 m/s 1 x y z Using the v squared formula for constant acceleration: s m m s m s m x g v v x g v v i f i f / 2 . 22 ) 20 )( / 8 . 9 ( 2 ) / 0 . 10 ( 2 2 2 2 2 2 2 = + = + = + =

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Physics 221 2004 S Final Exam Solutions Page 2 of 23 [78] The Figure at right shows the trajectories of three cannon balls fired simultaneously. In which order do they strike the ground? Neglect air resistance. (A) P Q R (B) R Q P (C) R P Q (D) Q P R (E) Cannot be determined without more information. The time it takes for the cannon ball to hit the ground along any given trajectory is equal to twice the time it takes to fall from a height of the peak of the trajectory. The order will thus be the same as the order of the peaks of the parabolas: R P Q
Physics 221 2004 S Final Exam Solutions Page 3 of 23 [79] Four blocks of mass m P =4kg, m Q =3kg, m R =2kg, m S =1kg are on a frictionless horizontal surface as shown on the figure below. The blocks are connected by ideal massless strings. A force F L =30N is applied to the left block and is directed to the left. Another force F R =50N is applied to the right block, and is directed to the right. What is the magnitude of the tension T in the string between m Q and m R . (A) T=14N (B) T=20N (C) T=30N (D) T=36N (E) T=44N Take right as positive. The acceleration of the whole train is (20N)/(10kg)=2m/s². This is the acceleration of the first two blocks which has a mass of 7kg so the net force on those blocks is 14N. The applied force is –30N therefore the force from the center string is +44N. The tension in the string is therefore 44N. [80] The body that is suspended by a massless rope has weight of 75N. The rope is pulling the body up at decreasing speed. Is the tension in the rope is (A) 75 N (B) greater then 75 N (C) less than 75 N (D) there is not enough information to distinguish between the answers A, B and C (E) 0 N The block is accelerating downwards so the net force on the block must be downwards. If the weight is 75N the upwards force due to the tension must be less than 75N.

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## This note was uploaded on 03/26/2008 for the course PHYS 221 taught by Professor Herrera-siklody during the Spring '08 term at Iowa State.

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examF_2004s_sol - PHYSICS 221 Spring 2004 Final Exam May 5...

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