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257_Physics ProblemsTechnical Physics

257_Physics ProblemsTechnical Physics - Chapter 9 P9.21...

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Chapter 9 259 P9.21 (a), (b) Let v g and v p be the velocity of the girl and the plank relative to the ice surface. Then we may say that v v g p is the velocity of the girl relative to the plank, so that v v g p = 1 50 . (1) But also we must have m v m v g g p p + = 0 , since total momentum of the girl-plank system is zero relative to the ice surface. Therefore 45 0 150 0 . v v g p + = , or v v g p = − 3 33 . Putting this into the equation (1) above gives = 3 33 1 50 . . v v p p or v p = 0 346 . m s Then v g = − = 3 33 0 346 1 15 . . . a f m s FIG. P9.21 *P9.22 For the car-truck-driver-driver system, momentum is conserved: p p p p 1 2 1 2 i i f f + = + : 4 000 8 800 8 4 800 kg m s kg m s kg b gb g b gb g e j b g ± ± ± i i i + = v f v f = = 25 600 4 800 5 33 kg m s kg m s . For the driver of the truck, the impulse-momentum theorem is F p p t f i = : F i i 0 120 80 5 33 80 8 . . ± ± s kg m s kg m s a f b gb g b gb g = F i = × 1 78 10 3 . ± N on the truck driver e j For the driver of the car,
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