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Homework Help: Physics Lab

  1. Sep 29, 2012 #1
    Okay, I am working on my physics lab, and I have a few questions regarding it.

    First, we are asked to make a prediction: "Predict how you think the presence of a rubber band will affect the force felt by the force probe from the rubber band compared to masses hung directly from the hook?" My lab partners and I thought that the force wouldn't transmit perfectly, because the rubber-band would pull upwards. Our data proved that that wasn't the case. Was are prediction truly wrong? Or, perhaps, did we set something up wrongly?

    Also, say I have a rubber band, and I hang it from some hook; I measure its length, and then I put some weight on it, and measure it again. How would I write the amount of stretch as a percentage?
  2. jcsd
  3. Sep 29, 2012 #2
    The prediction was truly wrong, as is evident from the experimental data.

    The percentage is (stretched length - original length) / (original length) x 100%.
  4. Sep 29, 2012 #3
    Okay, I am just having a hard time understanding why it was wrong. And why does subtracting the stretched from the original length, then dividing by the original length and multiplying by 100% give you the percentage of stretch. I am sorry, I just don't seem to understand how to interpret ratios.

    I am sorry if this information seems evident to you, but I have an dreadful intuition, and I always need to know why; sometimes it is quite bothersome.
    Last edited: Sep 29, 2012
  5. Sep 29, 2012 #4
    For the prediction, intuitively one might think that a force can only be "absorbed" by an acceleration. If a system is in a static equilibrium, then a force acting on one object will be "transmitted" by the object to its support (or "distributed" among multiple supports), where it will be cancelled by the reaction force; it is never absorbed by a stationary object.

    For the percentage, I am not sure what exactly your problem is. A percentage is a ratio of a special kind, whose denominator is one hundred. A ratio with one denominator can always be converted to an equal ratio with another denominator, thus every ratio can be expressed as a percentage.
  6. Sep 29, 2012 #5
    So, a rubber band in series will stretch twice the amount of a single rubber band with the same force being applied, is this a correct assumption?
  7. Sep 29, 2012 #6
    Each band will stretch the amount it would stretch individually - because the lower band stretches and the transmits the same force on the upper band. The total extension is, correctly, twice that. This is why the extension under force is measured as a ratio: it is always the same (for a given force), but the absolute extension depends also on the original length.
  8. Sep 29, 2012 #7
    Now, what would not make the lower band transmit the force perfectly to the upper band?
  9. Sep 29, 2012 #8
    Only some other force or motion. The other force might be static friction.
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