A perspective on motion sensors and free-fall

When an ultrasonic motion sensor is not aligned correctly with an object's motion, the geometry of the transmitted and reflected sonar signals leads to an underestimate of velocity. This is not a large effect under most teaching laboratory conditions, but compounding such an error in the production of acceleration data can produce surprisingly large overestimates. Presented here is an analysis of the general problem for free-fall experiments, followed by experimental data that show the relative importance of these errors. (C) 2015 American Association of Physics Teachers.