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Homework Help: Quick question on experimental errors

  1. Mar 25, 2016 #1
    1. The problem statement, all variables and given/known data

    A student is testing the kinematic equations for uniformly accelerated motion by measuring the time it takes for light-weight plastic balls to fall to the floor from a height of 3 m in the lab. The student predicts the time to fall using g as 9.80 m/s^2, but finds the measured time to be 35% greater. Which of the following is the most likely cause of the large percent error?

    A) acceleration due to gravity is 70% greater than 9.80 m/s^2 at this location.
    B) acceleration due to gravity is 70% less than 9.80 m/s^2 at this location.
    C) air resistance increases the downward acceleration.
    D) the acceleration of the plastic balls is not uniform
    E) The plastic balls are not truly spherical.
    2. Relevant equations

    basic kinematics and common sense
    3. The attempt at a solution

    I was able to narrow it down to E and D. If a ball is not truly spherical (and has more of a plate-like shape), then I thought that would increase surface area for the drag forces that would increase flight time. However, the solutions say this the correct answer is D. Could anyone please explain to me why I am wrong and they are right?

    Thanks in advance!
  2. jcsd
  3. Mar 25, 2016 #2
    Well!! However, non spherical "ball" it may be!! It won't have a plate like shape!!
  4. Mar 25, 2016 #3
    I mean it will have a bulging shape... more surface area... more drag?
  5. Mar 25, 2016 #4


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    Staff: Mentor

    A non-spherical shape is a small correction to the dominant effect of (D), which should be phrased as "air resistance reduces the acceleration" (but it also leads to a non-constant acceleration).
  6. Mar 27, 2016 #5
    OK thanks! That makes sense!
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