Time intervals measured with a stopwatch typically have an uncertainty of about 0.2 seconds, due to human reaction time at the start and stop moments. what is the percent uncertainty of a hand-timed measurement of (a) 5.5 s, (b) 55 s, (c) 5.5 min? time intervals measured with a stopwatch typically have an uncertainty of about 0.2 seconds, due to human reaction time at the start and stop moments. what is the percent uncertainty of a hand-timed measurement of (a) 5.5 s, (b) 55 s, (c) 5.5 min?