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Real Life Free Falls

  1. May 26, 2010 #1
    I was curious what some factors may contribute to free falling objects not falling at the same rate as would be expected in a vacuum. I remember from physics classes that two objects should fall at the same speed regardless of their weight. I know that surface area and air resistance plays a factor (like a feather vs a ball), but it doesn't seem to explain everything.

    What i am mostly thinking of is a truck going off of a jump, it always nose dives. Surface area I wouldn't think would play a factor since vehicles are mostly uniform. One explanation i have heard is that the front goes off the jump first, and therefore begins falling first. However, when you watch this in real life it usually takes a long time for the front to nose dive, i would think it would nose dive much faster if it were just a matter of the front falling first.

    Second example seems harder to explain and i dont know if i would believe it if i didn't just watch it on Mythbusters. They dropped a car from a crane and it nose dived. Then they corrected for weight making the front as heavy as the back. When they dropped the car with good weight distribution, it fell flat. Why should the weight make a difference?
  2. jcsd
  3. May 27, 2010 #2
    I'm not sure but I'll try.

    After experimenting (don't worry no car crashing) I think the difference originates from the fact that the crane as holding the car not above the mass centre and therefore the car started falling in an angle.

    my experiment was as follows:
    Take a pen (not Parker they tend to leak when they fall) and balence it between your tumb and index --> then drop it.
    Now, put something heavy (you could stick a pencil into a rubber) on one side and drop it once afterhaving held it at the original mass centre and a second time after having held it at it's new mass centre.
    What do you conclude?
  4. May 27, 2010 #3
    A large portion of a vehicle's weight is concentrated in the engine. This means the center of mass is located near the front of the vehicle. The "mostly uniform" surface area results in a center of pressure close to the geometric center of the vehicle, which is behind the center of mass.

    I suppose a car with a rear-mounted engine would go tail-first.
  5. May 27, 2010 #4
    good input, but i'm still a little lost. let me rephrase:

    If two objects are supposed to fall at the same rate in a vacuum regardless of their weight, then why would a falling object rotate due to the location of this center of gravity?
  6. May 27, 2010 #5


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    This would be simply solved by dropping an asymmetrically-weighted object in a vacuum.

    But there's one confounding factor that would have to be accounted for.

    When an asymmetically-weighted object is first dropped, the heavier end will tend to pull away first from whatever is gripping it. This will impart an initial rotation. If this factor were accounted for (such as a quick release), I'll bet the rotation will be largely mitigated.

    Then, if the experiment is repeated in a vacuum, I'll bet there will be no rotation at all.

    So, IMO rotation is due to:
    1] rotation imparted by release friction,
    2] air resistance.
  7. May 29, 2010 #6
    It just has to do with the location of the center of pressure with respect to the center of mass. If these two points do not coincide then a moment will be created and the object will rotate. If an asymmetrically weighted body is shaped so that the center of pressure and the center of mass are at the same point then there will be no moment and thus no rotation.
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