You drop from the window of your apartment at the 4th floor a 2.00 kg book to a
ID: 1283769 • Letter: Y
Question
You drop from the window of your apartment at the 4th floor a 2.00 kg book to a friend who stands on the ground at distance D=10.0m below. If your friend's outstretched hands are at a distance D=1.50m above the ground, (a) how much work Wg does the gravitational force do on the book as it drops to her hands? (b) What is the change U in the gravitational potential energy of the book-Earth system during the drop? If the gravitational potential energy U of that system is taken to be zero at ground level, what is U (c) when the book is released and (d) when it reaches her hands? Now take U to be 100J at ground level and again find (e) Wg.
Explanation / Answer
a) W = m*g*(H-h)
= 2*9.8*(10-1.5)
= 166.6 J
b) change in U = -W
= -166.6 J
c) Ui = m*g*H
= 2*9.8*10
= 196 J
d) Uf = m*g*h
= 2*9.8*1.5
= 29.4 J
e)
at a height 10m, Ui = 100 + m*g*H
= 100 + 2*9.8*10
= 296 J
at a height 1.5 m, Uf = 100 + m*g*h
= 100 + 2*9.8*1.5
= 129.4 J
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