Sammy L.

asked • 03/05/14

how many times a day do the minute hand of a clock make an angle of 90°?

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Philip P. answered • 03/05/14

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Philip P.

Steve S. (below) is right.  The minute hand actually takes 1 hour and 5 minutes to go around and catch back up with the hour hand.  This is because the hour hand continues to move as the minute hand moves.  Hence every 12 hours, the minute hand only overtakes the hour hand 11 times, 22 times in 24 hours.  Since it makes 2 right angle per cycle, that's 2 x 22 = 44 right angles per day.
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03/06/14

Philip P.

The period, or time it takes the minute hand to circle around and catch back up with the hour hand, is:
 
period = 65 minutes/60 minutes  = 1.08
 
The number of cycles per hour then = 1/period = 1/1.08 = .923
 
The number of cycles in 24 hours, then = (0.923)(24) = 22.153.  That's how you compute how many times the minute hand catches up the hour hand in 24 hours (round to 22).
 
Thanks for your question.  I've learned something today as well!
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03/06/14

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