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Homework Help: Can you decipher what my prof said regarding my hw question towards her?

  1. Oct 16, 2005 #1
    Can you decipher what my prof said regarding my hw question towards her!?

    My Q:> I did not acquire the accepted value for percent
    > difference that we are doing in lab for F spring and F rotation, was it
    > given, if so what is it?

    Response: Whatever the difference is, it is; discuss the difference in your report.
    Supposed to figure out the 2 values, based on values you
    measured or otherwise recorded in lab.

    So discuss the difference. But how do I get accepted values to use for percent difference?
  2. jcsd
  3. Oct 16, 2005 #2

    Tom Mattson

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    What exactly did you measure? What was the setup?
  4. Oct 16, 2005 #3
    I measured F rotation and F spring. F spring by by multiplying weight of my mass * 9.8 m/s^2 and F rotation by Slope(T^2)= = F/4*pi^2*m and solved for F. I just pluged them in to get my values for F rotation and F spring.
  5. Oct 16, 2005 #4
    bump, anybody?
  6. Oct 17, 2005 #5


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    Was the spring fixed to the axis and the mass rotated on a horizontal table while being attached to the spring?
  7. Oct 17, 2005 #6
    it was
  8. Oct 17, 2005 #7


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    Your instructor is correct.

    There is no such thing as "an accepted value for percent difference". There is an accepted value of a quantity, and there is an experimental value that you obtained, but there is NO accepted value of the differences. Each experiment will produce a range of value of a certain quantity. The difference will depend on the equipment, how well the experiment was done, random and systematic errors, etc. From what I can guess, you are asked to account for the difference between what you got, and what the accepted value is. This is important especially if the difference is large. Did you do something wrong? Did the equipment not perform the way it should when you were taking data? Did someone opened the door and let a gust of air in while you're doing it? Etc...etc.

  9. Oct 17, 2005 #8


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    Was there some instructions on how to change the speed of rotation as to keep a certain relationship between the radius of rotation and the period (or square of the period) of the rotation? If not, what was plotted on the axes of the graph?
  10. Oct 18, 2005 #9


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    Well, I still not sure exactly what you did, but it seems to me that the spring force should "theoretically" be the centripetal force that causes the rotational motion. Looking at the rotating mass one can also calculate this centripetal force. The difference between the two is what is under discussion. I said theoretically since one always have yo deal with friction and since the mass tends to slide outwards one would normally have a frictional force acting towards the centre of rotation. This frictional force is therefore helping the spring force to give the "real" centripetal force.
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