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How to Calculate Rotational Speed of Projectiles

The rotational speed of an object is simply the number of times the object rotates per unit of time. For example; an object that revolves five times per minute has a rotational speed of 5 rpm. Determining the rotational speed of a projectile, an object that is propelled by a force, presents a challenge because, to determine rotational speed, you must know how many times the object rotates per a specified unit of time. This is possible by recording the flight of the projectile and recording its time of revolution with a stop watch.

Things You'll Need

Time-lapse camera
Stopwatch

Instructions

- 1
  Mark the object with a bright-colored marker so the indicator is easily visible from a distance.
- 2
  Project the object into the air and record it with either a time-lapse camera or other recording device. Be sure you can clearly detect the indicator marker on the object.
- 3
  View the recording of the object after it reaches the ground. Using a stopwatch, record how long it takes the object to make one rotation. That is, how long it takes the indicator marker to move in a complete circle.
- 4
  Convert the duration of rotation time to seconds, if necessary. For instance; if the object took one and one half minutes to rotate, convert it to (60 seconds + 30 seconds) = 90 seconds.
- 5
  Divide the time duration of rotation into one. For example; for an object rotating at a rate of one rotation per 90 seconds, the expression becomes: (1 / 90) seconds = 0.011 rotations per second.
- 6
  Convert the measurement of rotations per second into other time intervals, such as minutes or hours, by multiplying the rotations per second by 60 to determine rotations per minute, then multiplying by 60 again to obtain the rotations per hour. For example; an object rotating at 0.011 rotations per second rotates at an equivalent rate of (0.011 * 60) = 0.666 rotations per minute and (0.666 * 60 minutes) = ~40 rotations per hour.

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