units lb*in x rpm don't work to get hp

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@rgunwaldsen wrote:

 

so 125 is wrong answer.

 

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I wouldn't call it wrong, but it is clearly inaccurate, yes,
The factor 63000 actually should be

And, no, I don't have the slightest idea about the underlying subject matter (otherwise I wouldn't have left the unnecessary and, when using units, incorrect conversion factor in my first answer). But after doing some research online to find out how horsepower is actually defined, it soon became clear how to calculate the correct factor.

This factor is necessary only when using a calculator or software which can't handle units. Fortunately Prime can handle units pretty well.

The factor starts at the definition of 1 horsepower to be 550 pound-feet per seconds

and converts inches to feet (12), seconds to minutes (60) and frequency(rev. per min) to angular frequency (2 pi). That way we can use torque in pound inches and revolutions per minute, multiply their values and divide by the conversion factor to get the result in horsepower.

Prime knows all these relationships between units and performs the necessary conversions itself when required.

Therefore, it is unnecessary and counterproductive to specify such conversion factors in Prime.

So we simply can use

And I guess that the more precise 124.95 are close enough to the expected 125 which we get with the inaccurate conversion factor 63000.

With a different perspective on the matter:

You sure could strip the units, use the formula as given and add unit hp at the end:

And now lets have a look if Prime could tell us what the conversion factor actually should be. Let's just collect all the units we used for dividing and add at last:

Voilà, you see that Prime knows its job and that you don't have to care about conversion factors as long as you use correct units.

And Prime can show you the result in any power unit and you actually don't must have any idea about the used conversion factors.