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Unformatted text preview: ergy is llU =':lU33 - Ul U33 -0 mgtly = (55x9.8 N)(32x3.5 mj:;;; 6004 kJ. (b) At the fifty-ninth floor, U59 - Ul (55x9.8 N)x (58 x 3.5 m) = 109 kJ. (c) Street level is th'e zero of "1 potentia! energy, UI == O. Note that the differences in potential energy between any two levels ai!e the same as in Example 8-1, e.g., U59 - U33 == (1091-60.4) kJ 49.0 kJ. = = = = 7. Show using Equation 8-2bthatthe potential energy difference between the ground and a distance h above the ground is mgh regardless of whether you choose the y-axis upward or downward. Solution
Equation 8-2b gives the potential energy difference for a constant force in the y direction, AU U(Y2) - U(Yl) = -FY(Y2 - yd. If you take the y-axis upward, with the ground at YI, the gravitational force is Fy = -mg, while a point a distance h above ground is Y2 h+ YI' Then AU = -(-mg)h = mgh. On the other hand, if the y-axis is downward, then Fy = mg but Y2 = -h + yl, so AU = -(mg)(-h) mgh is the = = Problem ;
.1 5. Find the potential energy of a 70-kg hi~...
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