M-K5: Stroboscopic Observation of Motion
 
PIRA: Unknown 

Equipment: Digistrobe, tennis ball, ball mounted on a wheel or other devices as desired. Set 22, Cabinet 9, Shelf 4 contains the strobe. The mounted ball can be driven by a motor. The ball mounted on a wheel is in Room 91.  The motor is in Set 4, Cabinet 2, Shelf 2 or Set 17, Cabinet 2, Shelf 1.

Warning: Do not leave the strobe light on very long, as its flashes can produce headaches or nausea in some persons.

Procedure: "I think it is instructive to show the acceleration in free fall by tossing a tennis ball up and catching it under conditions of nothing but stroboscopic illumination. The Strobotac type 1531A is suitable for viewing a thrown tennis ball.

"It is fun to play catch with a student under this type of illumination. Give the student the advantage by placing him with back to the light. You will have to operate with your back to the audience in order to see the ball in side light. If you face the light, you are partially blinded by the lamp itself, and the ball always presents the dark side to you. Do not persist in this type of experiment very long as many people develop headaches or nausea under conditions of flashing illumination.

"Always operate the equipment within the limits stated in the instruction manual or on the placards on the case.

"It is desirable to also point out that periodic motions can be falsely interpreted under stroboscopic illumination. As an example I suggest the rotation of a rather highly symmetrical object, such as a wheel with several spokes. Set the flash frequency a bit higher than for the tennis ball and start the wheel rotating. As the wheel slows down, it will appear to rotate first one way, stand still, rotate the other way, stand still, etc. However, the handle appears to rotate in the same direction over

most of the range of retardation (if the flash frequency has been well chosen).

"Violent motion of the hands under stroboscopic illumination while an assisting student turns the room lights on and off is interesting. The hand motion can be planned to demonstrate the unreliability of interpretation unless something of the nature of the motion is already known to the observer." ¾Percy Carr (1970)