49_Physics ProblemsTechnical Physics

49_Physics ProblemsTechnical Physics - 50 P2.69 Motion in...

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50 Motion in One Dimension P2.69 (a) yv t a t t t fi =+ = = + () 1 22 1 2 50 0 2 00 1 2 980 .. ., 4 90 2 00 50 0 0 2 ... tt + = t = + 2 00 2 00 4 4 90 50 0 2490 2 . Only the positive root is physically meaningful: t = 300 . s after the first stone is thrown. (b) t a t 2 2 1 2 and t = = 300 100 200 . s substitute 50 0 2 00 1 2 980 200 2 2 . . =( ) + ( ) v i : v i 2 15 3 = . m s downward (c) vv a t 11 200 980 300 314 = + = + = . . . a t 15 3 9 80 2 00 34 8 = + ( ) = . . . P2.70 (a) dt 1 2 1 2 . = 336 2 12 240 += . 336 4 90 2 40 2 = af 490 3595 2822 0 2 2 2 . t 2 2 359 5 359 5 4 4 90 28 22 = ± ( ) . . . t 2 359 5 358 75 00765 = ± = . . s s o == 336 26 4 2 . m (b) Ignoring the sound travel time, d ( )= 1 2 9 80 2 40 28 2 2 . . . m, an error of 682% P2.71 (a) In walking a distance x , in a time t , the length of rope A is only increased by x sin θ . The pack lifts at a rate
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