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Homework Help: Air resistance, maybe?

  1. Dec 17, 2006 #1
    1. The problem statement, all variables and given/known data
    Human reaction time is usually about .20s. If your lab partner holds a ruler between your fingers and thumb and releases it without warning, how far can you expect the ruler to fall before you catch it. (disregard air resistance g= 9.81 m/s^2)


    2. Relevant equations

    not sure what to use.

    3. The attempt at a solution

    I was thinking on using velocity? but I don't know, I don't think I have much info.:confused:
    the answer is 19.6 cm
    if anyone could please help me, set this up... I would truly appreicate it.
     
  2. jcsd
  3. Dec 17, 2006 #2

    cristo

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    Use the kinematic equations, namely s=ut+(at^2)/2, where s is displacement, u is initial velocity, t is time and a is acceleration.
     
  4. Dec 17, 2006 #3
    how far does something fall in .2 seconds?
     
  5. Dec 17, 2006 #4
    hm..I'm not sure, I'm still confussed...
     
  6. Dec 17, 2006 #5
    or wait...I think I go it.
    9.8 * 2?
     
  7. Dec 17, 2006 #6

    cristo

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    Use the equation s=ut+(at^2)/2. Here, a=g, you are trying to find out s, and t is the reaction time. Substitute these into the equation, remembering that the ruler starts from rest [so you know what the initial velocity is]. Does this help?
     
  8. Dec 17, 2006 #7
    ok, kind of there, but u- is initial velocity isn't that 0, at rest, when mulitplied to the time it goes to zero, doesn't it?
     
  9. Dec 17, 2006 #8

    cristo

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    Yes, that's right, so you're left with just one term on the right hand side.
     
  10. Dec 17, 2006 #9
    ok many thanks!!!
     
  11. Dec 17, 2006 #10
    ok so its: (0)(.20)+((9.8)(.20)^2)/2 =.196
    hm...what am I doing wrong?
     
  12. Dec 17, 2006 #11

    Integral

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    What are the units of your answer?
     
  13. Dec 17, 2006 #12
    oh well its supposed to be cm, do I have to convert or what?
     
  14. Dec 17, 2006 #13
    what units is the .169? cm?m?mm?
     
  15. Dec 17, 2006 #14
    oh I don't know, I mean I just did it in the calculator. well s and then m/s^2 isn't that m? blah, I'm lost.
    and in my answer its supposed to be 19.6 cm.
     
  16. Dec 17, 2006 #15

    cristo

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    Yes, your answer is 0.196 m... what is this value converted into cm?
     
  17. Dec 17, 2006 #16
    I move the decimal twice. I believe. Wow, I seriously need to memorize all the convertions.
     
  18. Dec 17, 2006 #17

    cristo

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    Yes, but instead of "moving the decimal place twice" you should memorise the fact that there are 100cm in a metre, so to convert from metres to centimetres you must multiply by 100.
     
  19. Dec 17, 2006 #18
    yes. thank you so much.
    wow there's a lot of stuff I need to learn. but I'll get there. thank you so very much, I truly appreciate it. =)
     
  20. Dec 17, 2006 #19

    cristo

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    You're welcome. Keep working and, like you said, you'll get there! Good luck!
     
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