Physics Photo of the Week

September 18, 2009

Student Kate Zeigler is driving Warren Wilson's team of Belgian draft horses - Curly Kate and Jenny.  They are discing the Warren Wilson vegatable garden in April, 2009 during the planting season.

During the developments of the industrial revolution 200 years ago, James Watt performed experiments with horses pulling to lift loads with hoists in order to measure the amount of work that a typical horse could perform in a standard unit of time.  He performed these measurements in order to compare the work capability of his steam engines with that of horses.  He found that a typical horse could produce 33,000 ft-lbs of work per minute.  In a quarry, for example, one horse attached to a rope and pulley could lift a 100 pound load of ore a distance of 330 feet in one minute.  This is also equivalent to lifting a 200 pound load a distance of 165 feet in the same minute.  The steam engines developed could lift 10-20 times the weights listed above in a minute, hence the steam engines would be rated 10-20 horsepower.  In physics textbooks, the horsepower has been assigned the metric or SI unit of 746 Watts or 746 Joules/sec.  Thus a horse on a treadmill geared to a generator should be able to power about seven 100 Watt lamps, assuming negligible friction.

Of course engine horsepower is very important in promoting high performance cars.  Engine power is usually measured with a device called a "dynamometer".  The dynamometer measures the torque by means of a mechanical brake.  The torque (foot-pounds) multiplied by the shaft angular velocity (revolutions/min) yields the power in foot-pounds/minute.

Even though engines can produce far more power than a horse, horses can go many places that motor vehicles cannot.  They don't get stuck in snow or mud; they can pull loads up much steeper hills.  Horses also offer companionship to the farmers as the students on the horse crew at Warren Wilson can readily attest.

"Plow Day" featuring farm draft horses had been scheduled at the Warren Wilson College Farm this Saturday.  However the event has been canceled due to excessive rain.  Watch for rescheduled times.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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