Simanaitis Says

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EVERY SO often, it’s good to think about basic physics and engineering matters such as torque, horsepower and, these days, their electrical equivalents. Being the internationalists we are, let’s discuss both good honest English units and their Système International equivalents.

Torque, or twisting force, is the easiest to describe. Imagine a 1-lb. weight pulling down the end of a 1-ft. stick. The twisting force at the other end is 1 lb.-ft. (not 1 ft.-lb. which is a unit of work, of which more anon).


The torque, or twisting force, of 1 lb.-ft.

The Système International unit of torque is the Newton-meter, where 1 Newton is the force that accelerates 1 kilogram at a rate of 1 meter/second2.  Few remember this, but it’s worth remembering that 1 Nm = 1.356 lb.-ft.

Sir Isaac

Sir Isaac Newton, 1642-1727, English natural philosopher, mathematician. Portrait by Godfrey Kneller.

Did you know that Isaac Newton was into alchemy—attempting to turn stuff into gold—as well as developing the calculus?

Horsepower, as seen in engine data, can be defined directly in terms of a powerplant’s torque: Horsepower = (Torque x RPM)/5252. And, in fact, this is how an engine’s horsepower is calculated on a brake dynamometer. This gizmo applies a brake to the crankshaft’s twist; hence the term “brake horsepower,” bhp.

The classic definition of horsepower goes back to real horses. James Watt wanted a way to compare a steam engine’s power with that of a draft horse. His investigation led to 1 hp equaling 550 ft.-lb./sec., that is, the power to lift 550 lb. a distance of 1 ft. in 1 sec., or any arithmetic combination thereof.

Thus, for instance, when a ballet dancer gracefully lifts a 137.5-lb. danseuse a distance of 2 ft. in 1/2 sec., he’s exerting….


This Tim Barker illustration of dancer lifting danseuse appeared in R&T, November 2003.

Let’s see, 137.5 lb. x 2 ft. is 275 ft.-lb. of work being done in that 1/2 sec. of lifting, equivalent to 550 ft.-lb. of work each second. That is—we assume here she doesn’t help with a little jump—he’s exerting 1 hp.

So much for these guys being considered wimps.

Watt. The SI unit of power is the watt, named in honor of you-know-who. James Watt, 1736-1819, was the Scottish inventor and engineer whose improvements in the technology of steam pretty much started the Industrial Revolution.


This statue of James Watt by Francis Legatt Chantrey resides in St. Paul’s Cathedral, London.

A watt can be defined as 1 joule/sec. But wait, what’s a joule?

The joule is a unit of energy, of passing an electric current of 1 ampere through a resistance of 1 ohm in 1 second.

There. That’s settled.

A joule is also approximately the energy required to lift a small apple a distance of 1 meter—which brings us back to Newton, doesn’t it? And, indeed, it’s also the energy expended in applying a force of 1 Newton through a distance of 1 meter.

Don’t worry about all this; life is “open book.” But I am curious who Joule was.


James Prescott Joule, 1818-1889, was an English physicist and brewer. My kinda guy.

We all know watts from lightbulbs, but did you know that the European Union has forced kilowatts to be used instead of horsepower? One kW = 1.34 hp, but, because of EU Directive 80/181/EEC, effective January 1, 2010, you can use the latter in the EU only “as supplemental.”

It was a lot more fun back in the old days when cars had imperial horsepower, or French cheval-vapeur, or German Pferdstärke, or Italian Bhp CUNY, these last apparently little ponies because the values were always so big. ds

© Dennis Simanaitis,, 2013

One comment on “POWER PLAY

  1. Bill Urban
    February 15, 2013

    After saying and writing “foot pounds” ~5,000 times over 40 years, I can’t believe I never researched pounds feet. I always thought, another toMAto toMOTo . . . Thanks prof. for the instruction.

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This entry was posted on February 13, 2013 by in Sci-Tech and tagged , , , , .
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