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Homework Help: Basic constant acceleration help

  1. Jan 26, 2009 #1
    Ok, I've been running over this problem for about an hour now, and I can't figure out where I'm going wrong. I'm a complete newb, so I'm sure it's something stupid, but I could use the help.

    1. The problem statement, all variables and given/known data
    An object with an initial velocity undergoes constant acceleration. Position information was collected at 4 poins and is shown in the table. Using Yn+1 - Yn = Vnt + (1/2)ayt2 determine the acceleration and fill in each box.

    edit : it screwed up my table, I hope it makes sense.

    Point #; Time (s); Position (m); Average Acceleration (m/s2
    1; 0; 1; n/a
    2; 1; 8; (answer here)
    3; 2; 25; (answer here)
    4; 3; 52; n/a

    2. Relevant equations

    Yn+1 - Yn = Vnt + (1/2)ayt2


    3. The attempt at a solution

    Here's one attempt I tried.

    25m - 8m = 7(1) + 1/2ay(1)

    ay = 20

    I've tried a few different attempts, I'm not sure I should show them all here. The problem I'm running into is that I think I need to plug in velocity figures, but I wasn't given any, so I messed around some, but the answers don't make sense, and aren't uniform.
     
  2. jcsd
  3. Jan 26, 2009 #2

    rl.bhat

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    At position 1 Yn = 1m, and Vn is the initial velocity.
    At position 2 Yn+1 = 8 m and t = 1s
    At position 3 Yn+1 = 25 m and t = 2s
    At position 4 Yn+1 = 52 m and t = 3s
    From position 1 and 2 you get 8-1 = V0 +1/2*a.........(1) Similarly wright the equations for positions 1 and 3, and 1 and 4. Solve the equations to find the acceleration.
     
    Last edited: Jan 26, 2009
  4. Jan 26, 2009 #3
    Ok, that all makes sense, but I'm still not sure what to do for initial velocity, since it wasn't given to us. Am I supposed to use some equation to find it?
     
  5. Jan 26, 2009 #4

    rl.bhat

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    Wright three equators. From the first equation we get Vo = 7-1/2*a. Use this value in other two equations to find a
     
  6. Jan 26, 2009 #5
    Thank you for your help, I figured it out earlier today, things are making more sense every day!
     
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