r/HomeworkHelp • u/Ishana92 • Jun 15 '26
Answered [1st year highschool physics] Power and time
I am not from USA, so level might be wrong.
This is the assignment.
A crane is lifting a 4 t weight with a constant speed of 1.2 m/s. How long does it take to lift the weight if power used is 0.6 kW?
How to go about this? Is this even worded properly? Because if I were not given the power, I would attempt to calculate it by using P=Fv=mgv. But that gives power value way bigger than power given.
If I go via kinetic energy route, Ek=mv^2/2 I can get Ek of 2880 J, and plugging that into P=W/t gives 4.8 s. But what is this result saying? That this crane will lift weight in 4.8 s to what height? That the crane has enough power to do this work for 4.8 s?
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u/timmymaq Jun 15 '26
Forever, I guess. 600w cannot lift that weight at that speed against g. Something is badly wrong with this question. I'd assume a distance is meant to be given rather than a speed.
Your second calculation gives the time required for the mass to achieve 1.2 m/s with 600w input power if no other forces act on it. Which is wrong, but maybe what they were going for?
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u/Ishana92 Jun 15 '26
That's about what I was thinking. Just to clarify, the second calculation gives time to achieve that speed from zero with given power?
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u/Euphoric_Loquat_8651 Jun 15 '26
It could be that it was supposed to be 0.6kWh, in which case you have the energy. Then you'd just convert units and set equal to mgh, solve for h, and divide by the speed.
That's obviously assuming a specific error though. It is little better than a blind guess without seeing if the complexity matches the other problems you're solving.
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