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Torque
Torque is rotational force. A simple example is the force you apply to a wrench to turn a bolt. The torque depends on the force you apply and on the length of the wrench. As you turn the bolt, the direction of the force changes to always act in a direction perpendicular to the axis of the bolt. When an engine turns a shaft, the shaft exerts rotational force, or torque, in the same way.
Torque Units
The amount of torque depends on the force and the lever arm. The force is the pressure you apply to a wrench and the lever arm is the length of the wrench. The Torque of 5.0Nm is a force of 5.0 Newtons at a distance of 1 meter from the shaft. A pound of force equals 4.45 Newtons so 5.0 Newtons equals 1.12 pounds of force. A meter equals 3.28 feet. The torque of 5.0Nm equals 1.12 x 3.28 or 3.7 foot-pounds.
Engine Torque
When an engine turns it delivers torque to a shaft and then to gears, pulleys or other loads. The torque is the force and, when applied for a period of time, it does work that is measured in horsepower. A typical car engine delivers over 100 horsepower and develops a torque of several hundred foot-pounds. An engine or motor that produces 5.0Nm or 3.7 foot-pounds is not very powerful and could, for example, be a small electric motor.
Torque/Speed
The torque an engine produces changes with the engine speed. Internal combustion engines develop no torque at standstill and little torque at very low speeds. As they reach several thousand RPM, they produce their maximum torque and then a smaller torque at their top speed. Electric motors have a starting torque and a higher torque at their rated speed. As a result, torque specifications for engines and motors give the torque at a particular speed. For a torque of 5.0Nm at 5000Rpm, this is written as 5.0Nm/5000 RPM.