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Homework Help: Clock and problem

  1. Jun 6, 2013 #1
    a clock was set 10 am 20 minutes 30 second ? what will be the angle between hour & second hand ??

    ans : 130.25° or 133° which one is correct? I'm confused
  2. jcsd
  3. Jun 6, 2013 #2
    The line of reasoning should be as follows:-

    It takes 12 hours for the hour hand to make a complete 360° rotation, i.e.

    12 hours-------> 360°
    ==> 1 hour -------> 30°
    ==> 60 min -------> 30°
    ==> 1 min -------> 0.5°
    ==> 20.5 min -------> 10.25°

    So, in a time period of 20.5 minutes, the hour hand would have traversed 10.25°.

    Now, when the clock shows 10 A.M exactly, the hour hand has traversed 300° (since for each hour, the hour hand traverses 30°. So, after 10 hours exactly, it has traversed 300°). And in a further 20 minutes and 30 seconds, i.e. 20.5 minutes, it would have traversed 10.25° further, as we have just calculated above. So, the total angular distance traversed by the hour hand is (300+10.25)° = 310.25°.

    Now, the second hand is at the exact position '6' in the clock (since it travels for a period of 30 seconds), and hence has traversed exactly half of the clock, thus travelling an angular distance of 180°.

    Therefore, the angle between the hour hand and the second hand would be (310.25 - 180)° = 130.25°.

    Hope that makes it clear! :smile:
  4. Jun 6, 2013 #3


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    Science Advisor

    The more interesting question is how the other answer was obtained.

    Offhand, it looks like a factor of 12 got erroneously applied to the seconds correction.
  5. Jun 6, 2013 #4
    The best explanation I could come up with is, that the OP first figured that at exactly 10:20 A.M, the hour hand would have traversed 310°. Then, for the remaining 30 seconds, he mistakenly calculated the angular distance traversed by the minute hand, which turns out to be 3°, and then added it to the angular distance traversed by the hour hand, giving him the final value of the latter as 313°. And since the second hand has traversed 180°, he obtains (313 - 180)° = 133°, which is false.
  6. Jun 6, 2013 #5


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    Science Advisor

    Nice. That fits.
  7. Jun 11, 2013 #6
    got it man...thanks :)
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