I want to measure accurately the time it takes for the drum of my clothes dryer

Question

I want to measure accurately the time it takes for the drum of my clothes dryer to make one rotation. Since the door on the dryer is made of heat resistant glass, I can see the drum rotate. I take my stopwatch, start it when a visible mark on the drum gets to the up-most position, and stop it when the visible mark on the drum returns to the up-most position after making 18 rotations. I estimate that my reaction time limits the accuracy of both my starting and stopping the stopwatch to 0.3 s. I measure that the 18 rotations take 17.8 s.

First, what is the error in my measurement of 18 rotations?(For the number use two digits to the right of the decimal point.)

Second, what is the time it takes for one rotation and its absolute uncertainty (same as absolute error)? (For the numbers, use three digits to the right of the decimal point.)

Explanation / Answer

Total time taken for 18 rotations = 17.8 s Error in measuring this time = 0.3 s The error in measuring the time for 18 rotations = 0.3 s Absolute error = T / T                         = (0.3 s) / (17.8 s)                         = 0.017 Time taken for one rotation = 17.8s / 18                                             = 0.99 s

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