You lift a 64.40-kg box from the floor onto a shelf 0.87m above the floor. If yo
ID: 1331357 • Letter: Y
Question
You lift a 64.40-kg box from the floor onto a shelf 0.87m above the floor.
If you lift the box with a constant speed, what is the force you exert on the box? Is it upward or downward? (You must answer both parts to get credit.)
upward
downward
What is the work you do on the box?(make sure you get the right sign)
What is the magnitude of the change in potential energy of the box? Does it increase or decrease? (You must answer both correctly to get credit.)
Increase
Decrease
Consider what your answers would be if you lower the box back to the floor. You do not need to enter the answers for this, but you do need to know what signs might change for the opposite case.
Explanation / Answer
The force aginst the gravitational attraction on the box.
ma = F - mg
0 = F - mg
Thus, the force exerted on the box is,
F = mg = 64.4 kg *9.8 m/s2 = 631.12 N, upward
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the work you do on the box is,
W = F*d = 631.12 N * 0.87 m = +549 J
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The change in potential energy is,
dU = mg[change in height] = 64.4 kg *9.8 m/s2*[0.87 m - 0] = +549 J
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