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CHAPTER 3 - 22 - a vertical...

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x h y v 0 O a vertical  =  a  sin 30° = (– 2.78 m/s 2 ) sin 30° =          – 1.4 m/s 2  (down) . 75. The horizontal velocity is constant, and the vertical velocity will  be zero when the pebbles hit the window.  Using the coordinate  system shown, we find the vertical component of the initial  velocity from v y 2  =  v 0 y 2  + 2 a y ( h  –  y 0 ) ; 0  =  v 0 y 2  + 2(– 9.80 m/s 2 )(8.0 m – 0),  which gives  v 0 y   = 12.5 m/s.   (We choose the positive square root because we know that the pebbles are thrown upward.) We find the time for the pebbles to hit the window from the vertical motion: v y  =  v 0 y  +  a y t ;   0 = 12.5 m/s + (– 9.80 m/s 2 ) t , which gives  t  = 1.28 s. For the horizontal motion we have x  =  x 0  +  v 0 x t ;   9.0 m = 0 +  v 0 x (1.28 s), which gives       v 0 x  = 7.0 m/s . Because the pebbles are traveling horizontally when they hit the window, this is their speed.
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CHAPTER 3 - 22 - a vertical...

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