Comparison of Four Different Methods for Measuring Projectile Velocity

Abstract

Four techniques for measuring the velocity of a free-falling body (a baseball) were compared for validity and reliability. The baseball, with two steel bolts attached, was made to fall free by releasing it from an electromagnet. The four methods studied were (1) stopwatch ( n = 61), utilizing a 0.1-sec. stopwatch and (2) line-pull ( n = 61), utilizing two lines of different length attached to the baseball. The shorter line when pulled taut triggered a switch to start a clock counter. The longer line when pulled taut stopped the clock. (3) For 16-mm motion picture photography ( n = 11), a Bell & Howell camera with Kodak Tri-X film (ASA 160) at 76 frames/sec. was used and for (4) strobophotography ( n = 10) flash intervals of 20, 40, and 60 msec. were employed to expose sequential images of the free-falling baseball on Polaroid Type 57 film (ASA 3000). Strobophotography gave the best agreement with theoretical velocity ( V0), followed by 16-mm motion photography, line-pull, and stopwatch methods, in that order. Strobophotography was also the most reliable of the four methods, having the lowest standard deviation for Vobs – V0. Next in order of reliability were line-pull, 16-mm motion photography, and stopwatch. Multiple-image strobophotography gave superior validity and reliability in measuring projectile velocity, while line-pull and 16-mm motion picture photography methods seemed adequate for most research applications as well as athletics and instruction. A stopwatch is suitable only when a rough estimate of a projectile's velocity is needed.