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Homework Help: Circular Motion lab problem

  1. Dec 4, 2006 #1
    This is a question pertaining to a lab we did. Ill try to describe it the best i can. What we pratically did was attach a rubber stopper to an end of a string, put the string to a hollow tube (like a torn apart pen), and tied a mass on the bottom. We had to hold on the tube and start swinging the rubber stopper horizontally by rotating the tube. The mass tied to the string caused a tension force. It is impossible to have the stopper on the string spinning perfectly horizontal because perpindicular forces are independent of each other. We then had to calculate the frequency of the stopper while changing some variables.

    Here is my question,

    For the greatest accuracy in this investigation, the tension force acting on the tsopper should be horizontal. In this context, what happens to the accuracy as the frequency of revolution of the stopper increases (with the other variables held constant)?

    i said as the frequency increases, the accuracy decreases, but i dont know how to explain why.
  2. jcsd
  3. Dec 4, 2006 #2
    Oh, i did a very similar lab to this one. Anyways, your answers is correct and i think it is so because as the stopper spins faster, it gets harder to count the revolutions and stop the timer at the exact time of the revolution occuring.
  4. Dec 5, 2006 #3


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    Homework Helper

    To make the stopper rotate you need to wave your hand to and fro. This motion will have the same period as that of the stopper. It is much easier to watch the motion of the operator's hand than to try and see what the stopper is doing (which is moving too quickly to see - the eye has great difficulty in focussing on it). It becomes much more difficult to count the revolutions as the speed increases.

    Yes, you are correct in thinking that the tension will point a bit upwards. The vertical component need to balance the weight of the stopper while the horizontal component supplies the centripetal force.

    The angle of the string below the horizontal should not increase with speed since the needed vertical component of the tension need not increase at a greater radius (weight of the stopper stays the same). The stopper will swing at a lower height though at a greater radius.
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