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B Fall Rate of Objects

  1. May 3, 2016 #1
    Hello -

    I have a question from a CSET practice exam that i'm having trouble understanding the provided answer.

    Q Two items are dropped simultaneously from a ten-story building. The items are shaped similarly and are relatively the same weight. Which of the following statements is correct concerning the rate at which the objects fall?
    1. The weight of the objects has no effect on the falling rate.
    2. The resistance of the air has no effect on the falling rate.
    3. The shape of the objects has no effect on falling rate.
    4. The distance of the fall has a major effect on the falling rate.

    The answer is 1. Can someone please explain why the other choices are incorrect. I understand that objects weight has no effect on the falling rate of acceleration, but why wouldn't the shape have an effect? The two objects are similar in shape, which would make sense why the it wouldn't have an effect but if one object was to differ in shape, then it would. The same could go for resistance of the air choice. The resistance would be roughly the same for both objects since they are the same shape and weight, but if it was to differ it would have an effect. Is there an easy explanation someone could provide?
     
  2. jcsd
  3. May 3, 2016 #2

    russ_watters

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    Staff: Mentor

    I strongly dislike this question. The constraints are vague and what kind of scientific term is "falling rate"? I might have answered choice #4!

    The choice of correct answer tells us the constraints: the objects are shaped/sized so that drag force is a function of weight (and speed), meaning that at all times you'll end up with the same acceleration with differently weighted objects.
     
  4. May 3, 2016 #3
    I agree with russ_waters. For example, the weight of the objects would certainly affect their falling rate, but not relative to each other is they are the same weight and shape. On #2, wouldn't they fall faster in a vacuum.

    Bad question.
     
  5. May 3, 2016 #4

    davenn

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    Science Advisor
    Gold Member

    but not relative to each other is they are the same weight and shape, and in an atmosphere

    devoid of atmosphere, shape difference is irrelevant :smile:


    Dave
     
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