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# Ch9 HW2 - WebAssign Ch9 HW2(Homework Current Score 58 out...

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Ch9 HW2 (Homework) Arthur Barfield PHYS 2211, section M, Fall 2008 Instructor: Jennifer Curtis Web Assign Current Score: 58 out of 59 Due: Friday, October 31, 2008 09:00 AM EDT Description Momentum and energy in collisions: Unequal masses; 2D collisions Instructions Reading: CH9 The due date for this assignment is past. Your work can be viewed below, but no changes can be made. 1. 9/9 points In outer space rock 1, with mass 6 kg and velocity < 3700 , -3200 , 1800 > m/s, struck rock 2, which was at rest. After the collision, rock 1's velocity is < 3200 , -2700 , 2100 > m/s. What is the final momentum of rock 2? 2 f = < 3000 3000 , -3000 -3000 , -1800 -1800 > kg · m/s Before the collision, what was the kinetic energy of rock 1? K 1 i = 8.151e7 8.15e+07 J Before the collision, what was the kinetic energy of rock 2? K 2 i = 0 0 J After the collision, what is the kinetic energy of rock 1? K 1 f = 6.582e7 6.58e+07 J Suppose the collision was elastic (that is, no change in kinetic energy and therefore no change in thermal or other internal energy of the rocks). In that case, after the collision, what is the kinetic energy of rock 2? K 2 f = 1.569e7 1.57e+07 J On the other hand, suppose that in the collision some of the kinetic energy is converted into thermal energy of the two rocks, where E thermal,1 + E thermal,2 = 1.10 10 7 J. What is the final kinetic energy of rock 2? K 2 f = 4.69e6 4.69e+06 J In this case (some of the kinetic energy being converted to thermal energy), what was the transfer of energy Q (microscopic work) from the surroundings into the two-rock system during

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the collision? (Remember that Q represents energy transfer due to a temperature difference between a system and its surroundings.) Q = 0 0 J Solution or Explanation sys = net t = 0, so the momentum of the system doesn't change. Therefore, 1f + 2f = sys,i If the collision was elastic then K 1f + K 2f = K sys,i If some kinetic energy is converted into the thermal energy of the two rocks, then K 1f + K 2f + E thermal, sys = K sys,i Although the system's temperature rises (associated with the increase in its internal or thermal energy), in the rapid collision there isn't time for any significant thermal energy transfer Q to the cooler surroundings. Eventually the system will cool down as a result of thermal energy transfer to the surroundings, but this will take a while. 2. 22/22 points Object A has mass m A = 9 kg and initial momentum A,i = < 16 , -8 , 0 > kg · m/s, just before it strikes object B, which has mass m B = 11 kg. Just before the collision object B has initial momentum B,i = < 5 , 7 , 0 > kg · m/s. Consider a system consisting of both objects A and B. What is the total initial momentum of this system, just before the collision? sys, i = < 21 21 , -1 -1 , 0 > kg · m/s The forces that A and B exert on each other are very large but last for a very short time. If we choose a time interval from just before to just after the collision, what is the approximate value of the impulse applied to the two-object system due to forces exerted on the system by objects outside the system?
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