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exam 2 spring 2010

# exam 2 spring 2010 - QUESTION 1 A 12 kg block moves in the...

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QUESTION 1* A 12 kg block moves in the x -direction at 28 m/s, and a 24 kg block moves in the y -direction at 8.0 m/s. Find the velocity of their center of mass. (a) ( V x , V y ) = (6.3 m/s, 1.6 m/s) (b) ( V x , V y ) = (2.8 m/s, 4.9 m/s) (c) ( V x , V y ) = (8.6 m/s, 2.6 m/s) (d) ( V x , V y ) = (9.3 m/s, 5.3 m/s) (e) ( V x , V y ) = (4.6 m/s, 1.4 m/s) QUESTION 2** A light automobile has a speed of 45 miles per hour in the + x direction and a heavy truck has a speed of 30 miles per hour in the - x direction. What is the relative speed s cm at which they approach one another in the center-of-mass system? (a) | s cm | < 75 mph (b) | s cm | = 75 mph (c) | s cm | > 75 mph QUESTION 3** This and the next two questions refer to the following situation:

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A 4.0 kg circular disk slides in the x -direction on a frictionless horizontal surface with a speed of 5.0 m/s. It collides with an identical disk that is at rest before the collision. The collision is elastic. Disk 1 goes off at an angle of 60° with respect to the x -direction. Disk 2 goes off at an angle of 30° with respect to the x -direction. Treat the disks as point objects and ignore the possible rotations of the disks. Which one of the following statements is correct? (a) Disk 1 has the greater kinetic energy after the collision. (b) Disk 2 has the greater kinetic energy after the collision. (c) The kinetic energies of the two disks are equal after the collision. QUESTION 4*** Find the speed of disk 2. (a) 2.5 m/s (b) 2.8 m/s (c) 3.3 m/s (d) 3.9 m/s (e) 4.3 m/s QUESTION 5* Find the sum of the kinetic energies of disk 1 and disk 2 after the collision. (a) 0 J (b) 25 J (c) 50 J (d) 75 J (e) 100 J
QUESTION 6* This and the next question refer to the following situation: In a Physics 211 laboratory experiment one end of a string is tied to a cart. The other end of the string is tied to a force probe that is fixed to the track. The cart is free to move on the track. Initially the string is slack. The cart is given a velocity v o and the magnitude of the momentum of the cart is 0.42 kg m/s. The experimental setup is shown below. The string becomes taut, and a force is exerted on the cart by the string. The string then becomes slack again. The force on the cart is measured as a function of time by the force probe. The data from the force probe are shown below. The integral of force on the cart with respect to time is 0.66 kg m/s.

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