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A math question

  1. Jan 15, 2010 #1
    Hello guys, could you please help me to solve this problem?

    A student normally needs 15 seconds for a 100m sprint. One day favorable wind conditionshelp her to increase her speed by 2km//h.
    What is her time now?

    A) aprx. 13,8 sec.
    B) aprx. 13,6 sec.
    C) aprx. 13 sec.
    D) aprx. 14,2 sec.


    Could you explain why ?
     
  2. jcsd
  3. Jan 15, 2010 #2
    What is her normal speed?
     
  4. Jan 15, 2010 #3
    Her speed, in m/sec is 100 m in 15 sec or 6.67 m/sec.

    First, convert that to km/hr.

    100 m is 0.1 km and there are 3600 sec in an hr.

    So, her speed in km/hr is 0.1 km/(15/3600 hr) which is 24 km/hr.

    Now, add the increase due to the wind: 24 km/hr + 2 Km/hr = 26 km/hr

    That's 26000 m in 3600 sec. Or 7.22 m/sec.

    How long does it take to run 100m at the rate of 7.22 m/sec?

    100 m / (7.22 m/sec) = ~13.8 sec, so the answer is A).
     
  5. Jan 16, 2010 #4
    Thank you very much!
     
  6. Jan 16, 2010 #5

    Mark44

    Staff: Mentor

    It is against the policy in this forum to provide complete answers to posters' questions.
     
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