Simply stated, power is a rate of doing work. And work is defined as exerting a force through a distance. So, if you exert a force of one pound through one foot of distance you've performed one foot-pound of work. (This is NOT the same thing as a foot-pound of torque! The units happen to be the same, but they're quite different things.) If you pushed with one pound of force through one foot of distance in one second, the power you've applied is one foot-pound per second. If you did it in half a second, then the power is doubled to two ft-lb/sec. If it took you two seconds, that would be half a foot-pound per second.
The units may vary, but power is always force x distance / time. One horsepower is 550 ft-lb/sec or 33,000 ft-lb/min.
So far, we've only discussed power in linear terms. But engines and motors are rotary devices. Without getting into details, for rotary devices power equals torque x angular velocity. When dealing with English units, one horsepower equals (torque (in foot pounds) x rpm)/(33,000 ÷ 2π) or (torque x rpm) / 5252.
All a dynamometer does is apply a load (linear force or torque) to an engine or vehicle. If you know both the force/torque and speed/rpm, it's simple to calculate power.
When you look at the Nebraska tests, you'll see that horsepower is measured in two different ways: PTO and drawbar. PTO horsepower is simple to measure, while drawbar horsepower is not. But of course drawbar horsepower is for most purposes the more important measurement, since it measures the actual rate at which work can be done pulling an implement. Which is of course what really matters.
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Today's Featured Article - The Ferguson System Principal An implement cutting through the soil at a certain depth say eight inches requires a certain force or draft to pull it. Obviously that draft will increase if the implement runs deeper than eight inches, and decrease if it runs shallower. Why not use that draft fact to control the depth of work automatically? The draft forces are
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