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Homework Help: How stretching a rubber band, affects the range of a shot?

  1. Apr 8, 2017 #1
    1. The problem statement, all variables and given/known data
    I did an experiment on how stretching a rubber band, affects the range or distance of a projectile shot.
    Variable list:
    • Independent Variable
      • Amount of stretching
    • Dependent Variable
      • Range or distance
    I created my catapults on my own and used two different rubbers bands and compared them

    2. The attempt at a solution
    Then I calculated the average, however i wanted to show that there is a linear relationship between the stretch and the range, but i dont know much about it. so can you help in determining whether what i did was right or wrong?
    upload_2017-4-8_15-37-35.png upload_2017-4-8_15-37-1.png

    Then I graphed the average and produced a line of regression, but it didnt pass through the points so its not linear... Also i wondering whether i can get the potential energy, is it possible?
    upload_2017-4-8_15-38-27.png upload_2017-4-8_15-38-59.png

    Attached Files:

    Last edited: Apr 8, 2017
  2. jcsd
  3. Apr 8, 2017 #2


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    Gold Member

    That's not necessarily true. You need error bars. Your line may not pass through the average, but does it pass through any of the measured values? Can you estimate what they are? If not, plot all three points instead of the average and see what you get. Also, the angle of launch is critical for the range. What did you do to make sure that the angle was the same in all launches?

    Also, your plot is unconventional. The independent variable is on the horizontal axis and the dependent variable on the vertical, not the other way around. For a linear plot it doesn't make much difference, but for other kinds of dependence it could be confusing and disorienting to the viewer.
  4. Apr 8, 2017 #3
    Alright, i will try inputting all the measurements and see what happens, however, i did add the error bars but for some reason, it's only visible when u zoom in. I used ploty https://plot.ly/create/ to get the line of reg.
  5. Apr 8, 2017 #4
    I tried all measurements, it is still not linear. :(

    I think i might have done it wrong :|:cry:
  6. Apr 8, 2017 #5
    is it alright if the best line of fit misses the one point? will it considered linear?
  7. Apr 8, 2017 #6


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    Gold Member

    Yes and no. Let me explain. You need to understand how well your theory of linearity matches the experiment that you undertook. "Linear" in your case means that the range is proportional to the stretch for all values of the stretch. So answer me this, why did you start stretching at 0.10 m? What if you started at 0.01 m or 0.001 m?
  8. Apr 8, 2017 #7


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    2017 Award

    Staff: Mentor

    Why do you expect a linear relation?
    If you know the launch angle and the projectile mass, you can make some estimate about the kinetic energy of the projectile.
    Error bars should be based on the variations between the trials, not based on the measurement uncertainty of the individual launches. Your estimate for the average is much more uncertain than a millimeter.

    If your measurement uncertainty is really a single millimeter as shown in the table (I doubt that), you should give the measured ranges with one more digit.
  9. Apr 8, 2017 #8
    fixed it, thank you for replying :redface:

    oh good point, ill try doing it and thank so much!

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