Prony Brake (1M50.10)

Description:

Named after Gaspard de Prony, the Prony brake measures torque produced by an engine or in this demo, a student. A student cranks on the handle for one minute against a frictional force. Knowing the drum circumference, the number of turns (and thus distance traveled), the force, and the time, horsepower can be calculated (about 0.25 hp max).

Procedure or Operation Notes:

Ask for three student volunteers from the first or second row.

Ask the first student to crank on the handle as fast as possible to complete as many revolutions as possible in 60 seconds.

Note: The red weight weighs 20 pounds as shown on the large spring scale and we will assume that the first student will supply an additional 50 pounds of force on the scale. This is not exact and the scale will oscillate during each revolution. The blue tape mark indicates 70 pounds of force on the spring scale.

Ask the second student volunteer to set a countdown timer for 60 seconds, either on a cell phone or with a stop clock.

Ask the third student to count the number of revolutions produced by the first student in 60 seconds. The large yellow disk has a black band for increased contrast and easier counting.

The total distance is the drum circumference (1.625 feet) times the number of revolutions. Given the force (50 pounds) and the time (60 seconds), the total Power produced by the student is:

Power = (Force)(drum circumference)(# of revolutions)/(time) = (50 pounds)(1.625 feet)(# of revolutions)/(60 seconds).

Example: A student makes 75 revolutions (rounded up) in one minute, find the Power produced.

Power = (50 pounds)(1.625 feet/revolution)(75 revolutions)/(60 seconds) = 101.6 ft-lbs/sec = 0.18 hp. Where 1 horsepower (hp) equals 550 ft-lbs/sec.

Producing 1 horsepower would require turning the handle over 400 times in one minute!