A student investigates a rotating turntable in the laboratory
PHYSICS · Multi-part · A Level · Physics · Measurements and their errors · Measurements and their errors
Problem
A student investigates a rotating turntable in the laboratory. A small piece of reflective tape is fixed to the rim so that a photogate counts each time the tape passes. The photogate records 250 flashes during a time interval of 20.0 s. Simultaneously, a separate linear speed probe placed tangentially at the rim of the turntable reads 1.20 m s⁻¹. All measurements carry the uncertainties listed below.
• Number of flashes ±1 count
• Time interval ±0.10 s
• Linear speed reading ±0.05 m s⁻¹
Assume the tape remains at the outer edge and that the table rotates without wobble.
(a) Determine the average angular speed of the turntable during the 20.0 s interval.
(b) Using the simultaneous linear speed reading, calculate the radius of the turntable.
(c) Propagate the given measurement uncertainties to obtain the absolute uncertainty in the radius. State the radius with its uncertainty using proper significant figures.
(d) The student suggests moving the tape halfway toward the center to reduce the flash count rate. Qualitatively explain how, if at all, this change would affect the radius determined by the method used in parts (a)–(b), assuming the linear speed probe remains at the rim.