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Phys298HW2SolnSp08 - Phys 298 Brown Solutions to graded...

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Phys 298 Brown Solutions to graded problems from week 2 homework. Ch. 2 #78 This is a free fall problem. Given/Known: " y = 1.30 m " t = 0.22 s a y = g , t 0 = 0, y 0 = 0 I choose to call down my positive y-direction. You don’t have to make the same choice, but you have to be consistent with your choice once you make it. The way I want to approach this is to find the speed/velocity of the balloon as it enters from the top of the window. Then I can figure out how far the balloon has fallen from rest above the window to reach this speed. I use: y = y 0 + v 0 y t + 1 2 a y t 2 " y # y 0 # 1 2 gt 2 = v 0 y t " v 0 y = $ y # 1 2 gt 2 t = 1.3m # 1 2 (9.8m/s 2 )(0.22s) 2 0.22s = 4.83 m/s Knowing that the balloon is moving at 4.83 m/s when it reaches the top of the window, use the equations of kinematics to figure out how far it has fallen from rest. It is like a separate problem where now 4.83 m/s is the FINAL velocity and 0 is the initial velocity. Given/Known: v y = 4.83 m/s, v 0 y = 0 a y = g , y 0 = 0 Find y . Start from the equation of kinematics that doesn’t involve time: v y 2 = v 0 y 2 + 2 a y ( y " y 0 ) v y 2 = 0 + 2 g ( y " 0) = 2 gy # y = v
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