[Physics] Mass Effect on Slingshot Motion

elasticityenergyforceskinematicsprojectile

For my physics class (I'm a high school student), we created slingshots. Our task is to predict the distance a projectile, launched from a slingshot using surgical tubing, would land. We aren't given the mass of the projectile until the day of testing, so we would have to use a graph to find out the distance. My partner and I conducted a series of tests comparing mass to distance from a given pullback length ($55$ cm). The angle the projectile is launching from is $54^\circ$. K is not linear, but for our set pullback length our K value is $100$ (N/m). Our results show a cubic, or possibly quadratic graph. Why is this? Why would the relationship between mass launched and distance follow this shape? Here is a graph of our data:

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Best Answer

I congratulate your teacher on posing a fascinating intellectual challenge. So here's my guess (as a former physics student turned (medical) physician) constructed by analogy with the typical analysis of simple harmonic motion. In that situation with a spring constant of K and the force dependent on instantaneous displacement from the center of the oscillation, there is a resonant frequency that is a function of the mass and the spring constant. You are essentially looking at one-half of such an oscillator, so I think there would be an optimal ratio of mass to spring tension that would achieve the maximal transfer of energy from tubing to projectile. The resonant frequency of such a system would be one half the inverse of the time from release to the midpoint of the slingshot. (This of course assumes you can measure the time from release to crossing the slingshot "Y", but I'm sure you have some high-speed cameras around for that task, right?)

Do some searching on this website or on Wikipedia for the calculation of resonant frequency in "simple harmonic motion" and measure the spring constant of your slingshot. See if you can pull all these together for an A+ scientific report describing your experimental results!