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Kinematics - identifying and describing types of motion

  1. Mar 21, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-3-21_22-46-33.png

    2. Relevant equations

    acceleration equation:
    a = v2 – v1 / t2 – t1

    3. The attempt at a solution
    Can someone please tell me if my answers make sense. Thanks in advance :)


    21.
    upload_2016-3-21_22-47-32.png

    22.
    to calculate the velocity, i determined the slope of the tangents, postioned at 0.50s, 0.75 s, and 1.00 s

    upload_2016-3-21_22-48-12.png
    Using this information, i created a velocity time graph for the orange
    upload_2016-3-21_22-51-52.png

    Acceleration:

    a = v2 – v1 / t2 – t1

    a = 8.5 m/s (down) – 6.7 m/s (down) / 1.00 s – 0.75 s

    a = 7.2 m/s2 (down)
     

    Attached Files:

  2. jcsd
  3. Mar 21, 2016 #2

    haruspex

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    Not bad, but which of all the available datapoints for velocity should be the most reliable?
     
  4. Mar 21, 2016 #3
    what do you mean? are you referring to the velocity i calculated in my time-velocity data table? and what do you mean by most reliable? arent they all reliable?
     
  5. Mar 21, 2016 #4

    haruspex

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    They're not wonderfully reliable because you are drawing tangents by eye and finding the velocities from their slopes. I can see, for example, that your tangent at 0.5s is a bit too steep.
    There is one completely reliable velocity that you have not used. You do not even have to draw a tangent to find it!
     
  6. Mar 22, 2016 #5
    i dont understand :P
     
  7. Mar 22, 2016 #6

    haruspex

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    Which bit don't you understand, the inaccuracy inherent to the method you have been told to use to find velocities, or what data point you have not used?
     
  8. Mar 22, 2016 #7
    both
    i was just told to draw tangents and then calculate the slope of each tangent to determine the velocity
    here's an example from my lesson:
    upload_2016-3-22_2-42-30.png
    upload_2016-3-22_2-41-55.png

    upload_2016-3-22_2-43-24.png
    upload_2016-3-22_2-43-44.png
     
  9. Mar 22, 2016 #8

    haruspex

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    Yes, I understand that, but you must realise that drawing tangents by eye like that is open to substantial error. The tangent you drew at 0.5s looks to me more like the tangent at 0.55s.
    Since you are following a worked example, I suppose you cannot do much better than that. But had I prepared the example I would have taken the velocity at time 0 as an obvious known value. Note that using that value together with your t=1 tangent gives a rather more accurate value for g.
     
  10. Mar 22, 2016 #9
    ok i understand, should i fix the tangent at 0.5 s, will that make it more accurate?
     
  11. Mar 22, 2016 #10

    haruspex

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    Yes, a bit.
     
  12. Mar 22, 2016 #11
    ok thank you.
     
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