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# Homework6-help - 9.17 The system has nonzero initial...

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9.17 The system has nonzero initial momentum p i x . As Dan (D) jumps backward off the gliding skateboard (S), the skateboard will move forward in such a way that the final total momentum of the system p f x is equal to p i x . This conservation of momentum occurs on the system. Solve: We have () () ( ) Sf Df S Di SD x xx mv m mv += + . Hence, () ( ) ( ) ( )( ) f D 5.0 kg 8.0 m/s 50 kg 5.0 kg 50 kg 4.0 m/s x v + ( ) f D 3.6 m/s x v ⇒= 9.26 . Model: Model the rocket as a particle, and use the impulse-momentum theorem. The only force acting on the rocket is due to its own thrust. Visualize: Please refer to Figure P9.26. Solve: (a) The impulse is J Fd t = = area of the F x ( t ) graph between t = 0 s and t = 30 s ( )( ) 1 2 1000 N 30 s = = 15,000 N s (b) From the impulse-momentum theorem, p f x = p i x + 15,000 Ns. That is, the momentum or velocity increases as long as J x increases. When J x increases no more, the speed will be a maximum. This happens at t = 30 s. At this time, mv f x = mv i x + 15,000 N s ( ) f 425 kg 425 kg 75 m/s 15,000 N s x v + f 110.3 m/s x v 9.35 Model: This problem deals with a case that is the opposite of a collision. The two ice skaters, heavier and lighter, will be

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## This note was uploaded on 04/18/2010 for the course PHYS ? taught by Professor Zhou during the Spring '10 term at Georgia State.

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Homework6-help - 9.17 The system has nonzero initial...

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