EK no explanation problem 376 / 379

[GRod18]

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EK's answer for this one is correct answer is D.

A stationary 10 kg box is lifted and placed at rest on top of a 10 m shelf. The move takes 2 s. A student argues that the minimum average power is greater than 500 W. The student is:

Answer:Wrong because the force on the box could be twice the weight for the first half of the trip, and zero for the 2nd half of the trip?


379. A fork lift moves a 500 kg box from one 4 m shelf to a second 4 m shelf in 100s. What is the minimum power required by the forklift?

answer is 0 W because P = mgh/t and h did not change. What I don't understand is since W = Force x displacement we are still moving the object a distance D right? Why is the power still 0? Are we only suppose to consider work by gravity?

 

[Rabolisk]

After thinking about this over dinner, I have come to the conclusion that these two questions were meant to test a very simple concept, but turned out to be tricky. For the first one, the minimum average power isn't greater than 500W, it is 500W. For the second question, the power required by the forklift to "lift" is 0, although it is true that you need some sort of power to move anything.

Bad questions more than anything, IMHO. I don't think your understanding of the material is the problem here.

 

[GRod18]

Thank you so much, sorry you had to divert some blood to the brain during dinner time lol...