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Acceleration has nothing to do with mass, but

  1. Sep 10, 2009 #1
    In my physics class, we did a free fall lab where we had three different balls dropped from different heights. The balls were of varying masses--a steel ball, a golf ball and a hollow plastic ball. Now I'm asked why the acceleration due to gravity is less for the plastic ball than the others. I have to explain why I think the plastic ball would behave like this and the other balls would not.

    I think it has to do with the force and mass. I know acceleration is independent of mass, but the hollow plastic ball has less mass compared to the steel and golf ball, thus less force is acted on it. I don't think my physics professor would think this is a legitimate answer though. I'm actually sort of confused whether he is convinced that somehow acceleration due to gravity is different for the lighter ball or it's some kind of trick question.I would greatly appreciate is someone could confirm what I think or correct me if I'm wrong. Thanks!
     
  2. jcsd
  3. Sep 10, 2009 #2

    jtbell

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    Staff: Mentor

    Hint: Is gravity really the only force acting on the balls while they are falling?
     
  4. Sep 10, 2009 #3
    Aerodynamic drag?
     
  5. Sep 10, 2009 #4

    berkeman

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    Staff: Mentor

    Yep!
     
  6. Sep 11, 2009 #5
    You've got to be careful here with how you describe it (I have a habit of misunderstanding posts so this may not be what you actually meant).

    Your description (in the seconds part) seems to describe that the effect of gravity is reduced for the lower mass but this just isnt so. The force due to gravity is lower, but it will accelerate at the same speed due to lower mass.

    The acceleration due to gravity is constant for everything, it is totally independent of mass. It is the same for the plastic ball, cannonball, tennisball, etc etc etc. We know this is true when two things of different masses dropped in a vacuum.

    The NET acceleration downwards is different due to forces acting to resist the motion, such as air resistance.

    So gravity always provides the same downwards acceleration, but forces aciting upwards create upwards accelerations. When you add then together you get a net downwards acceleration that is lower than the value due to gravity alone.

    I know it seems like nitpicking, but it is kind of a critical distinction to make.
     
  7. Sep 12, 2009 #6
    @ exparrot : Yep, this is a basic confusion many students have and theres nothing wrong with you if you have small concepts not understood well, you are atleast making an effort to undrstand it. So cheer up.

    The acceleration due to gravity remains the same for all objects, no matter what their mass, size.

    Acceleration due to gravity is the rate at which a body will speed downwards towards the earth when falling down freely. The mass of the three objects was different, but the acceleration they experience would remain the same. Thus they would speed downwards with the same rate which would continously increase their velocity as there is a constant continous rising change every instant in the velocity as the constant acceleration acts on it.

    The only difference it would make would be in the force created by the acceleration and the corersponding mass of each ball. This force is the Weight of the objects. And the weight would vary as the mass, so lighter ball having lower weight and vice versa.

    Note that though the mass would vary in the equation F = m . a, the acceleration ie the rate of speeding up ( downwards) wil lremain the same for each ball irespective of the mass.
     
  8. Sep 12, 2009 #7
    Watch this feather-drop by Apollo-15 astronauts on the Moon. Gallileo was right.
    http://video.google.com/videoplay?docid=6926891572259784994# [Broken]
     
    Last edited by a moderator: May 4, 2017
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