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Kinematics problem!

  1. Oct 16, 2007 #1
    1. The problem statement, all variables and given/known data

    A rocket powered sledge accelerates at 35ms-2 and takes 1.6s to pass through a 60m section of a test track. Find the speed at which it entered the section what was its speed when it left?

    2. Relevant equations

    I think this is the equation to use x=ut + 1/2at2 and than use a=v-u/t to get v ?


    3. The attempt at a solution

    a= 35m/s
    t= 1.6s
    u=?
    v=?
    x= 60m

    I got wierd answers so theres no point putting them up!

    please show your working

    thanks
     
  2. jcsd
  3. Oct 16, 2007 #2

    malty

    User Avatar
    Gold Member

    What exactly do you mean by a weird answer? Please put workings up, because your method seems right.
     
  4. Oct 16, 2007 #3
    when i make 'u' the subject of the forumal i got 65.5 ( which is wrong) and for 'v' I got 121.5

    I think my teacher said the answer was 9.5 for u and v i dont know,

    what did you get ?
     
  5. Oct 16, 2007 #4
    I know its an 'easy' question but i think i am just getting the equation wrong
     
  6. Oct 16, 2007 #5

    malty

    User Avatar
    Gold Member

    Teacher is right, just remember that experience is the best teacher so by making this mistake and figuring out where you made it you've learned more, than me simply showing you the solution.

    If you want you can show me you workings step by step, but as far as I can tell your eq is right your just calculating it incorrectly.How did you get 65.5?
     
  7. Oct 16, 2007 #6
    okay,

    x= ut + 1/2 at2

    rearranged to give;

    u= 1/2at2 + x/t ( this is were I think I am going wrong , rearranging it?)

    than just plug in the numbers. = 65.5

    OR
    alternatively you can 'show' your working out=)
     
  8. Oct 17, 2007 #7
    Alternatively, I found you can use simultaneous equations:
    (v+u)/2=s/t...So, v+u=75 (i)
    Also, v=u+at...So, u=v-56 (ii)
    Now, we have 2v-56=75, and v=65.5...u=9.5
    That's a simpler, but more bothersome method...
     
    Last edited: Oct 17, 2007
  9. Oct 17, 2007 #8
    Your transposition is a little faulty... If s=ut+1/2at^2...Then u = (s-1/2at^2)/t...What you found was your final velocity, v...Since, s=vt-1/2at^2...v=(s+1/2at^2)/t....Go back over your steps in rearranging the equation, you'll see the mistake.
     
    Last edited: Oct 17, 2007
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