Power problem again

[silverice]

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

The Answer saids the student is wrong, because the force on the box could be twice the weight for the first half of the trip, and zero for the second half of the trip.

Can someone please explain the answer? How does the force on the box could change from double to zero in 10m? I'm very confused. Thanks for the help.

 

[Captain Sisko]

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

The Answer saids the student is wrong, because the force on the box could be twice the weight for the first half of the trip, and zero for the second half of the trip.

Can someone please explain the answer? How does the force on the box could change from double to zero in 10m? I'm very confused. Thanks for the help.

Not sure about your solution either. It's just adding energy. If if says average then I don't see how that's wrong. Maybe someone smart like milski can weigh in

 

[peacefulheart]

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

The Answer saids the student is wrong, because the force on the box could be twice the weight for the first half of the trip, and zero for the second half of the trip.

Can someone please explain the answer? How does the force on the box could change from double to zero in 10m? I'm very confused. Thanks for the help.

please see the old post for this question:


http://forums.studentdoctor.net/showthread.php?t=752539

 

[Captain Sisko]

please see the old post for this question:


http://forums.studentdoctor.net/showthread.php?t=752539

That thread discusses a wording issue, but according to the laws of thermodynamics entropy always increases. So I say that ek is wrong and that the minimum power is greater than what was quoted above.

 

[silverice]

please see the old post for this question:


http://forums.studentdoctor.net/showthread.php?t=752539

Thank you. Do you guys agree on its just a question testing on a simple concept? Should I just ignore the answer explanation then? I have absolutely no idea what they mean by " the force on the box could be twice the weight for the first half of the trip, and zero for the second half of the trip. "

 

[milski]

Thank you. Do you guys agree on its just a question testing on a simple concept? Should I just ignore the answer explanation then? I have absolutely no idea what they mean by " the force on the box could be twice the weight for the first half of the trip, and zero for the second half of the trip. "

The simple concept and the explanation don't contradict each other. In short, for conservative forces (gravity, electrostatic) the average power is just the total work divided by the total time. What they're trying to say in the answer explanation is that you don't have to maintain the same power over the whole interval. Their example corresponds to tossing the object up - during the first second you apply force larger than its weight and accelerate it up, during the second part you don't apply any force at all and it slows down to a full stop at the end of the second due to gravity. You can have all sorts of variations in the force applied on the object - like lifting it faster, then slower, maybe even letting it drop a bit - at the end the work to get it 10 m high is fixed, so for a fixed time the average power is fixed as well.

That's fairly similar to average velocity - if you move 1000 m in 10 s, your average speed is 100 m/s. It does not matter if you started slow and got faster, or maintained constant speed or anything else.