Mechanical energy used per unit time is called mechanical power. Mechanical power is the rate at which work is done. Mechanical power is expressed in units of joules/sec (joules/s) or a watt (W) in the mks system, and feet - pounds force per second (ft-lbf/s) or horsepower (hp) in the English system. Mechanical power can be calculated using the following mathematical expression.

Because work can be defined as force times distance, we can also use the following equation:

where:

One horsepower is equivalent to 550 ft-lbf/s and 745.7 watts. Because in the equation above d divided by t is the same as velocity, an alternate description of power is as follows.

where:

When using equations 5-6 or 5-7, you must either assume force and velocity are constant or that average values of the force and velocity are used.

Example 1: A pump provides a flow rate of 10,000 gpm. The pump does 1.5 x 10⁸ ft-lbf of work every 100 minutes. What is the power of the pump in hp?

Example 2: A boy rolls a ball with a steady force of 1 lbf, giving the ball a constant velocity of 5 ft/s. What is the power used by the boy in rolling the ball?

Example 3: A race car traveling at constant velocity can go one quarter mile (1455 ft) in 5 seconds. If the motor is generating a force of 18901bf pushing the car, what is the power of the motor in hp? Assume the car is already at full speed at t=0.