Posted by MarkB_MI on December 25, 2020 at 11:05:20 from (174.230.13.56):
In Reply to: Re: H.p. at lower rpm posted by Ronnie Budd on December 25, 2020 at 09:21:46:
> Now , go back and read the rest of the history on how James Watt came up with the "formula" that we have been using for a hundred years. The measurements he did resulted in odd numbers that wouldn't easily convert. So, they rounded them off to come up with the constant of 5252 that is used as the divisor in our modern horsepower formula. So yes, it is "theoretical".
The formula for power has not changed since Watt's time. It is P = fv, where P is power, f is force and v is velocity. When dealing with rotational rather than linear work, the formula becomes P = 2πTn, where T is torque and n is the rotational speed. That "constant" of 5252 is not really a constant but rather a conversion factor. Working in the metric system, there is no need for a conversion factor. But using English units, one horsepower equals 33,000 ft-lb/min. 33,000 divided by 2π is 5252.
Power is not a "theoretical" value. It is a derived value, meaning it is calculated from other values (torque and speed) as it cannot be directly measured. To say power is theoretical would be like saying electrical resistance is theoretical, just because it has to be calculated from voltage and current.
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