How much work is done by gravity on a 50-kg box that is pushed up a 5 m ramp to a new elevation 2 m higher than its initial height?
A. 2500 J
B. 1000 J
C. -1000 J
D. -2500 J
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Answer:
C) -1000 J
Reasoning:
The work done by gravity is independent of the force provided by the people or the pathway. The work done by gravity is positive when an object moves downhill and is negative when an object moves uphill. In this problem, the box is moved uphill, so the work turns out to be negative. Because the term "against" is used, the value should be positive. The magnitude of the work done can be determined from the change in potential energy, because the box starts and finishes at rest, so KE did not change.
Wg = = ΔPE = - mgh = - (50 kg)(10 m/s2)(2.0 m) = - 1000 J
Changing the sign to positive, to correspond with "against gravity," makes C the best choice.
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Questions:
1. The problem never used the term "against" so I'm not sure if this free standing question originally came from another passage or something, so that confused me.
2. I'm confused what they mean by: "Changing the sign to positive, to correspond with "against gravity," makes C the best choice."
I thought they are saying the answer is negative?
A. 2500 J
B. 1000 J
C. -1000 J
D. -2500 J
------------------------------------------------------------
Answer:
C) -1000 J
Reasoning:
The work done by gravity is independent of the force provided by the people or the pathway. The work done by gravity is positive when an object moves downhill and is negative when an object moves uphill. In this problem, the box is moved uphill, so the work turns out to be negative. Because the term "against" is used, the value should be positive. The magnitude of the work done can be determined from the change in potential energy, because the box starts and finishes at rest, so KE did not change.
Wg = = ΔPE = - mgh = - (50 kg)(10 m/s2)(2.0 m) = - 1000 J
Changing the sign to positive, to correspond with "against gravity," makes C the best choice.
-------------------------------------------------------
Questions:
1. The problem never used the term "against" so I'm not sure if this free standing question originally came from another passage or something, so that confused me.
2. I'm confused what they mean by: "Changing the sign to positive, to correspond with "against gravity," makes C the best choice."
I thought they are saying the answer is negative?