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Instantaneous Velocity: drag racing problem

  1. Feb 4, 2012 #1
    The figure shows a graph of actual position-versus-time data for a particular type of drag racer known as a "funny car."
    [​IMG]

    Estimate the car's velocity at 2.0s

    Equation:
    v = m/s



    I thought I knew how to solve this, and I made a tangent line at the curve and 2s, and when I do the slope at different points on the tangent I keep getting somewhere in the field of 50m/s. However, it keeps telling me I'm wrong. What should I be doing differently?

    Thank you for your help!
     
  2. jcsd
  3. Feb 4, 2012 #2

    BruceW

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    your answer looks ok to me. what keeps telling you that its wrong?
     
  4. Feb 4, 2012 #3
    The program I have to use for class (Mastering Physics). It says express the answer with proper units and 2 significant figures, but whether I put in 50m/s or 50.0 m/s, it's still saying it's incorrect?
     
  5. Feb 4, 2012 #4

    SammyS

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    I get a somewhat steeper slope. ≈ 65 m/s

    Have you tried entering scientific notation?
     
  6. Feb 5, 2012 #5
    Thanks!

    67 m/s ended up being the correct answer. I redrew my tangent line, and took another slope and I ended up getting an answer extremely close to that, so my tangent line was just off before. Only problem is now, I have to do this again for t=4s. I have, once again, exhausted most of my attempts trying to get the correct answer. What am I doing wrong?? Here is my tangent line for t=4:
    [​IMG]

    I am getting a slope around 80m/s. Is my tangent line really that off?

    Thank you so much for your help!
     
  7. Feb 5, 2012 #6

    SammyS

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    That's at t = 4 seconds !
     
  8. Feb 5, 2012 #7
    I know that. :) This is for the second question, which is for the velocity at t = 4s. The first question was t = 2s. I got that answer for t = 2s correct now, it was 67 m/s. But now I can't get the correct answer for t = 4s.
     
  9. Feb 5, 2012 #8

    SammyS

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    How do you figure that slope to be 80 ?

    It's much more like ≈250/2 = 125
     
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