Rotational speed of an object is often confused with other measurements such as surface speed and tangential speed. The rotational speed of any object is based on the time it takes it to complete one full circle. The standard international measurement is called a hertz, which is one cycle per second. The hertz is also used to measure the frequency of other cyclical phenomenon, such as sound waves, radio waves and alternating currents in electricity. It always refers to the time it takes a periodic phenomenon to complete one full cycle, as measured in seconds. In most western countries, the measure of rotation in machine components is done in rotations per minute, or rpm.
- Skill level:
- Moderately Easy
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Things you need
Observe the object that you want to measure the rotational speed of. With the stopwatch, time how long it takes for it to make one full rotation.
Convert the time it took for a full rotation into units of measure. If the object you timed took 24 seconds to make a full rotation, the speed is properly expressed in hertz or rpm.
Divide one by however many seconds a full rotation took to get the speed in hertz. If the object took 24 seconds to make a full rotation, the calculation would be: 1/24 = 0.041667 hertz.
Divide the number of seconds it took for the object to make a full rotation by 60 to express the rotational speed in rpm. In the example, the calculation is: 24/60 = 0.4rpm.
Tips and warnings
- The hertz measurement is used for large, slowly rotating bodies. In very quickly rotating machine parts, rpm is more easily expressed. To translate hertz into rpm, simply multiply by 60. To translate rpm into hertz, divide by 60. For high-velocity machine parts, sophisticated measuring devices are used to calculate rotational speed.
- Do not confuse rotational speed for surface speed or tangential speed. The rotational speed on your car's tires is the time it takes each to rotate one full turn. The surface speed is how fast the outer surface of your tire is going. The larger the wheel, the faster the car will go, based on the same rotational speed. The outer edge of a disc must cover much more distance to make the same rotation in the same time as the inner edge. This is why when you get different-sized tires, you have to recalibrate your speedometer.
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