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Unformatted text preview: kim (jkk547) HW06 Tsoi (58020) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points An object having an initial momentum that may be represented by the vector below strikes an object that is initially at rest. Which of the following sets of vectors may represent the momenta of the two objects af- ter the collision? Note carefully: The original vector above and the following vectors are all drawn to the same length scale. 1. 2. 3. 4. correct 5. 6. 7. Explanation: There is no external force for the two-object system, so the total momentum is a constant. From the choices, applying the vector sum- mation for the momenta of the two objects, we can easily identify the correct choice. The figure below shows the sum of the x- and y-components of the vectors which repre- sent the correct answer. initial momentum The horizontal vectors add to be the same length as the vector presented in the question. The vertical vectors cancel, as expected, since there is no vertical momentum. 002 10.0 points A rubber ball strikes a sidewalk at an angle with respect to the horizontal and bounces off the sidewalk, again making an angle with respect to the horizontal. The direction of the impulse vector on the ball is 1. horizontal. kim (jkk547) HW06 Tsoi (58020) 2 2. vertically downward. 3. vertically upward. correct 4. at an angle with respect to the vertical. 5. at an angle with respect to the horizon- tal. Explanation: The force that the ground does on the ball is vertically upward, so the impulse on the ball is vertically upward. 003 (part 1 of 2) 10.0 points Given: Each railroad car has a mass of 97000 kg. Four coupled railroad car moving with a speed of 4 . 2 m / s collide and couples with seven other coupled railroad cars moving in the same direction with an initial speed of 1 . 8 m / s. What is the speed of all eleven cars after the collision? Correct answer: 2 . 67273 m / s. Explanation: Let: The mass of a railroad car be m = 97000 kg, the speed of the faster 4 cars be v 1 = 4 . 2 m / s, the speed of the slower 7 cars be v 2 = 1 . 8 m / s, and the speed of all 11 cars after the collision be v f . From conservation of momentum p = 0 4 mv 1 + 7 mv 2 = 11 mv f v f = 4 mv 1 + 7 mv 2 11 m = 4 (97000 kg) (4 . 2 m / s) 11(97000 kg) + 7 (97000 kg) (1 . 8 m / s) 11 (97000 kg) = 2 . 67273 m / s . 004 (part 2 of 2) 10.0 points How much energy is lost in the collision? Correct answer: 7 . 11098 10 5 J. Explanation: The initial kinetic energy is K i = mv 2 1 2 + 7 mv 2 2 2 = 4 (97000 kg) (4 . 2 m / s) 2 2 + 7 (97000 kg) (1 . 8 m / s) 2 2 = 4 . 52214 10 6 J ....
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- Fall '09