I did a video (30 fps) of the motion of of a mass suspended vertically by just one spring in one side. This spring is held by a rope connected to a fixed rod, as shown in the picture.

enter image description here

From the bottom to the top of the picture, we see, a bar of aluminium , another mass, a plastic ruler connected to the spring. The spring is connected to the rope on the other ending. Sorry for the bad resolution of the picture.

Then I used the $Tracker$ to plot the motion of the mass - plot of points - tracks points from 20 to 20 frames - and I got some results that I did not expected what puzzles me.

Could anyone explain me the motion of "equlibrium position" - why it goes up and down?

enter image description hereenter image description here

Thanks in advance.

  • 3
    $\begingroup$ Hi JoseAf, could you label the image of the apparatus a little? I see a mass suspended on a spring that seems to be bracketed by an acrylic cylinder. This spring then seems to be connected to another spring, and then to a piece of rope attached to the support. Is this correct? If so, are the two spring constants the same? Could you also include the frame-rate of the video? Also, is this effect reproducible? How sure are you that it's just not something in the setup jiggling? A video would be helpful if you have it, frankly :) $\endgroup$
    – Philip
    Nov 15, 2020 at 16:41
  • $\begingroup$ It is difficult to see what is happening in your photo. Can you provide a diagram to show the various parts and how they are moving? What is the purpose of the clamp just below the mass? The string exits at the top and bottom of the photo - what is happening in those places? Where is the "fixed rod"? $\endgroup$ Nov 15, 2020 at 16:44

1 Answer 1


The following may be a useful approach.

I'm not entirely sure of the details of your set-up, but the deviations shown on your figures seem to be in the millimeter range ($0.1 \times 10^{-2}$ m) which are quite small given the size of the equipment. It might be worth fixing the mass in place, then checking the measurements to see what kind of noise/uncertainty you have in the measurement system under the best conditions. This might help you localize and better understand the source of the fluctuations. It may turn out to be the noise characteristic of the measurement system itself.

I hope this helps.


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