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Homework Help: Physics Laboratory (Projectile Motion)

  1. Mar 11, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm doing this experiment in physics laboratory (I) about projectile motion and I'm concerned about some of the results (range) I'm getting. I'm using spherical projectiles about 3.0 cm wide (diameter).

    Here's the thing, with the launch angle from 10 to 20 (thus, from 80 to 70) I'm getting everything as the theory predicts.

    However, when I launch it at 45 (max range) or even 40/50 I get a maximized range but I get deviations of the balls in the y axis, meaning that |Δy|>0 (and not ≠0 as it should be).

    2. Relevant equations

    I've asked my professor and even suggested the cause of it being the aerodynamics of the balls along with the friction force but he denied it and didn't give me a straight fulfilling answer.

    3. The attempt at a solution

    My 'theory' about these deviations is that the farther a ball travels, the more time it's subjected to the friction force (obvious) and so, along with variations of pressure (due to its movement) the ball is forced to deflect (rotate) to the left or right. Again, I'm not sure this is true.

    I could really use some feedback from you guys :tongue:
  2. jcsd
  3. Mar 11, 2012 #2


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    Do you mean you get deviations in where the ball lands? (I'm assuming that's what you are measuring). You were saying the ball is deflected to the left or right. So that means the ball lands somewhere which is not along the direction it was launched in? And so the 'y axis' is horizontal and perpendicular to the launch direction?
  4. Mar 11, 2012 #3
    Exactly and I'm sorry for not being clearer. What do you think?
  5. Mar 11, 2012 #4


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    I can think of another possible reason that doesn't involve friction. But your explanation sounds reasonable too. I think maybe you need to explain your theory more. You've said the friction force deflects it to the side, and that the longer its in the air, the more this deflection is. So starting with a small deviation in the position, or spin, or whatever of the ball, then in what way does friction act to magnify this, and how does this affect the lateral position of the ball?
  6. Mar 11, 2012 #5
    What's your reason not involving friction?

    Those deflections lead me to believe that the ball rotates. I don't know what's the dynamics behind it while it's in the air, I mean, I do but not with accurate precision. I imagine that while the projectile is in the air, friction as long with pressure variations play a part by introducing spin onto the ball, just maybe.

    Other cause that can explain the desviations may be due to the launcher. Perhaps, when it's initially launched, the launcher gives the ball a small but signficant rotation and as it progresses along its path, it contributes to deflecting from the x axis. The farther it travels, the more it deflects.

    What do you say about this one?
  7. Mar 12, 2012 #6


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    the idea that the frictional force causes the ball to curve to the side is a reasonable explanation. I think maybe your professor said no because there is a simpler and more likely explanation for why Δy is greater when the ball spends more time in the air. For the simpler explanation, imagine there is no air friction, and the ball doesn't spin, and there's no air pressure differences. So now we can think of the motion of the ball depending on only the initial velocity of the ball when it is launched. So if there is some slight deviation in the direction the ball was launched, then how will this affect the motion of the ball afterwards?
  8. Mar 13, 2012 #7
    Assuming that the projectile launcher isn't perfect, which is more likely, we would have the ball follow a slight deviated path. This really sounds more likely of happening then the conjugation of friction,air pressure and sudden rotation, it's true.
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