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What equation to use for simple kinetics quesitons?

  1. Mar 12, 2012 #1
    For the questions below, which kinematic equations would I use? I have already tried these on my own but can not find a way to match the answers with the answer sheet, the answers on the answer key are RIGHT, as they are approved by my teacher.
    Okey so the questions are
    1) A baseball is thrown up vertically at 25m/s. What velocity will the ball have after:
    a)1.0s
    b)2.0s
    c)3.0s

    2) A soccer ball is kicked straight up at a speed of 30m/s. How long will it take to reach a height of 5.0m?
    It would be great if you showed me how you got your answers, if not, I will really appreciate the equations, THANKS!
     
  2. jcsd
  3. Mar 12, 2012 #2

    tiny-tim

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    Welcome to PF!

    Hi Alipepsi! Welcome to PF! :wink:

    For both questions, use one of the standard constant acceleration equations.

    If you're still stuck, show us what you've tried, and then we'll know how to help! :smile:
     
  4. Mar 12, 2012 #3
    Thanks
     
  5. Mar 12, 2012 #4
    Tiny, I'am confused =(
    I've looked over the equations but can not figure out what equation to use. PLEASE HELP ME I have a test after march break =S
     
  6. Mar 12, 2012 #5

    tiny-tim

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    For 1), you have u t and a, and you want v

    For 2), you have u s and a, and you want t

    So which of the constant acceleration equations has only u t a and v,

    and which has only u t a and s ? :smile:
     
  7. Mar 12, 2012 #6
    a=vf-vi/t

    Then just derive it :P
     
  8. Mar 12, 2012 #7

    tiny-tim

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    Yup! :biggrin:

    (And the other one? :smile:)​
     
  9. Mar 12, 2012 #8
    Vf^2=vi^2+2a(d)
    v=sq root 30^2-2(9.81)(5)
    v= 28.3m

    THEN

    Solve for time using the equation: t=(v-vf)/a

    answer is 0.17seconds ;D

    THANKS FOR YA HELP MAN! :D
     
  10. Mar 12, 2012 #9

    tiny-tim

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    You're welcome! :smile:

    But quicker would have been s = ut + 1/2 at2 :wink:
     
  11. Mar 12, 2012 #10
    I see, can you help me with how to know what equations to use for questions. Like I was confused for what equation to use for the second problem. Do I look for all the values I'am given with the values in all teh equatiosn then pick the one? Ik my quesitons is silly :P
     
  12. Mar 12, 2012 #11
    I always write down a list of all the given values when I start a problem. Then I compare them to the equations that I know.
     
  13. Mar 12, 2012 #12
    So you have to derive for t right?

    This is what I got:

    t= sq root s-ut/0.5a

    This is correct? Really need to know this asap
     
  14. Mar 12, 2012 #13
    yes, you'd have to solve for t
     
  15. Mar 12, 2012 #14
    did I derive for t correctly there?
     
  16. Mar 12, 2012 #15
    you'd have to use the quadratic equation to solve for t in that case.

    notice how t= sq root s-ut/0.5a still has a t ion both sides of the equation.
     
  17. Mar 12, 2012 #16
    If I was to use quadratic, then how would I get my A, B, and C values from solving that equation?
     
  18. Mar 12, 2012 #17
    well the equation is also this:

    1/2 at2 + ut - s = 0

    so if t is the variable, what are the constants?
     
  19. Mar 12, 2012 #18

    jtbell

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    You can check for yourself by substituting your formula for t back into the original equation to see if everything cancels out algebraically. Or you can use it to calculate t, then substitute it and your given quantities back into the original equation to see if everything cancels out numerically.
     
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