Even casual car enthusiasts know the term horsepower, a measurement of how much power an engine can produce. The higher the number, the more likely a car will smoke a Tesla in a race.
But what exactly is horsepower, and how much do horses actually have?
According to Science Focus, the concept of horsepower was developed in the 1700s by engineer James Watt (the eponym of the unit of power) and intended to entice people used to the workload of horses to switch to Watt's newly improved steam engine. By stating that an engine could outwork a horse by multitudes, people had a way of comparing the novel machine to their existing equine laborer and be incentivized to actually buy it. After all, if a machine could do the work of several horses, why not?
To figure out what “horsepower” actually meant, let's remember that power is defined as the amount of energy transferred (a.k.a. "work") during a particular unit of time. Watt claimed a horse was capable of turning a large mill wheel 2.5 times every minute—from there, he calculated the work the horse put into the job in an average day, a unit he called horsepower. Watt concluded that one horsepower equaled 33,000 foot-pounds per minute, or the power needed to lift 33,000 pounds 1 foot high in one minute.
The best way to think about this for the non-mathematically inclined is to picture a horse working to move a 33-pound weight 1000 feet in the air in one minute. That’s one horsepower.
This is largely the measurement used in modern cars, along with metric horsepower, which measures how much power it would take to raise a 75-kilogram mass 1 meter over one second.
So one horse should have one horsepower, right? Not exactly. It depends on how much work the horse can do in a given period of time. Data taken from the 1925 Iowa State Fair indicated that one horse could reach nearly 15 horsepower, though that's not sustainable. One horsepower refers to how much work a horse can put in for an average workday. The higher the horsepower, the higher the rate.
For cars, this can mean being able to accelerate faster, though not necessarily from a dead stop. (For that, you need to look at torque.) A car’s horsepower also typically refers to its peak output.
So would you ever want a lower horsepower number? Sometimes. In a car, it might improve fuel economy. In a horse, it might make for a more pleasant ride.
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