You drop a 2.50 kg textbook to a friend who stands on the ground at distance D =

Question

You drop a 2.50 kg textbook to a friend who stands on the ground at distance D = 10.0 m below. Your friend's outstretched hands are at distance d = 1.50 m 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 textbook-Earth system during the drop?

(c) If the gravitational potential energy of that system is zero at ground level, what is its potential energy U when the textbook is released?

(d) If the gravitational potential energy of that system is zero at ground level, what is its potential energy U when the textbook reaches the hands?

(e) How much work Wg is done on the textbook by its weight as it drops to your friend's hands if U is 100 J at the ground level.

(f) What is the change U in the gravitational potential energy of the textbook-Earth system during the drop if U is 100 J at the ground level.

(g) Find U at the release point when U is 100 J at the ground level. Incorrect: Your answer is incorrect. J (h) Find U at the hands when U is 100 J at the ground level.

Explanation / Answer

(a) W = mg*h = 2.5*9.8*(10-1.5) = 208.25 J

(b) Change = -W = -208.25 J

(c) P.E = 0+mg*h = 2.5*9.8*10 = 245 J

(d) P.E = 0 + mg*h = 2.5*9.8*1.5 = 36.75 J

(e) Work done remains the same = 208.25 J

(f) Change is also the same = -208.25 J

(g) P.E = 100 + mg*h = 345 J

(h) P.E = 100 + mg*h = 100 + 2.5*9.8*1.5 = 136.75 J

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