The time rate at which work is done by a force is said to be the power due to the force. If an amount of work W is done in an amount of time force, the average power due the force during that time interval.
The SI unit of power is the joule per second. This unit is used so often that it has a special name, the watt (W), after lames Watt, who greatly improved the rate at which steam engines could do work. In the British system, the unit of power is the foot-pound per second. Often the horsepower is used. Some relations among these units are,
Perhaps because they appear on our utility bills, the watt and the kilowatt-hour have become identified as electrical units. They can be used equally well as units for other examples of power and work or energy.
For example, the truck in exerts a force F on the trailing load, which has velocity vat some instant. The instantaneous power due to F is the rate at which F does work on the load at that instant. Saying that this power is the power of the truck is often acceptable, but we should keep in mind what is meant: Power is the rate at which the applied force does work.
CHECKPOINT 5: A block moves with uniform circular motion because a cord tied to the block is anchored at the center of a circle. Is the power due to the force on the block from the cord positive, negative, or zero?