MadSci Network: Physics

Re: How would you calculate the time needed to lift an object?

Date: Wed Oct 28 14:04:27 1998
Posted By: John Balbach, Post-doc/Fellow, Physics, National Institutes of Health
Area of science: Physics
ID: 907683453.Ph

Naturally, the difficulty lies in the definition of the power of the
motor.  Is it the power that it draws from the wall, or is it the power
that it is capable of delivering?

I have little experience with motors, but I can make some qualified 
guesses.  Comparing with radio frequency transmitters, if the box says
it delivers 1000 Watts, then I expect to get 1000 Watts of output from
that transmitter.  I am certain that it draws much more than that out of
the wall because the box has to have large fans to keep it cool.  Some of
the energy coming from the wall is converted to heat, and is lost.

Given that comparison, I would expect that the 5hp motor gives 5hp as its
output, and that the shorter time is correct.

If you feel the need for another example, you might consider an automobile
engine.  The horsepower rating of an engine is not very meaningful if it
refers to the power consumption.  There is a much better rating for that
quality (MPG).  I would assume that the horsepower refers to the power 
output of that engine.


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