What is horsepower and what does it measure?
The Sunday Nation
Nairobi,
15 May 2011
David Macharia “was a little confused after reading [the] article [on power measurements]”. Therefore, he asks a straightforward question: “What is horsepower and what does it measure?”
The straightforward answer is: a horsepower is the power of a horse, that is, amount of work that a horse can do in a given duration. It measures the power of a machine. However, that is not very helpful, so let’s dig a little deeper.
The concept of horsepower was first used scientifically by the father of steam engines, James Watt. Before mechanical engines were invented, people used animals (mainly horses and bulls) to do hard tasks. The most common horse-driven machines were rotary mills and water hoists (I am reluctant to call them pumps).
These machines had and axel connected to a long arm where the horse would be tied. The animal then walked around in a circular path thereby rotating the axel and driving the machine. This presented the perfect market for James Watt’s steam engine.
But to convince customers about the benefits of the engine, he needed to draw a comparison to the horse. So, he worked out the amount of work the animal was doing as follows.
He estimated that a horse could maintain a force of 180 pounds for one hour. When tied to a 12-foot arm, it could turn the mill 144 times in one hour. Now scientifically, work is defined as the product of the distance and force.
The distance walked by the animal round one cycle is simply pi-D; that is twice the length of the arm multiplied by 3.14; that is, about 75 feet. Therefore, 144 turns come to 10,850ft.
Thus the work done by the horse in one hour is about 1,953,000 foot-pounds (10,850 ft x 180 lb). The rate of doing work is therefore about 540 ft.lb per second. This is the definition of one horsepower.
Now all this has been calculated using imperial units. We need to convert it to metric measurements using the following equivalents: one foot is about 30cm or 0.3m and one pound is about 0.454kg.
Punching these into a calculator yields that one horsepower is equivalent to about 73.5kg.m/s. For the avoidance of doubt, I must emphasise that this is NOT kilogram PER metre per second; it is kilogram-metre per second!
Before, proceeding to the next step, I should point out that while in the imperial system the unit of work is expressed as a distance-force (ft.lb), the metric notation reverse the order to force-distance (kg.m). Nevertheless, the two symbolisms obey the basic definition of work – it doesn’t matter which way the multiplication is done!
In the metric system, the standard unit of force is the newton (while the name is written with a lower case ‘n’, the symbol is a capital ‘N’). A one-kg weight is equal to about 10N. Therefore, one horsepower (73.5kg.m/s) comes to about 735Nm/s.
Due to the immense contribution that James Watt made to the study of power, he was honoured by naming its unit after him. One newton-metre per second is called one watt (W). Therefore, one horsepower is equivalent to 735W. QED.
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