Question about liquids being dropped from tall buildings

Discussion in 'Physics & Math' started by notPresidentAndrew, Jan 14, 2003.

  1. notPresidentAndrew Banned Banned

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    A penny dropped from the top of the Sears Tower would continue down at an increasing speed. It would become very hot and would hit a car at the foot of the tower like a bullet.

    What would happen to a gallon of water if poured from the Sears Tower? Would it all evaporate because of the extreme heat it would gain? My bet would be that the water would break up into several small droplets and evaporate, but I would like to know if I am wrong.
     
    Last edited: Jan 15, 2003
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  3. Chagur .Seeker. Registered Senior Member

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    Huh? You're pulling my leg (or something close to it) aren't you?

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  5. notPresidentAndrew Banned Banned

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    No. I have a question posted above.
     
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  7. Chagur .Seeker. Registered Senior Member

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    Don't you realize:

    1. A penny would never fall that fast.

    2. It would depend on the temperature and humidity.

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  8. Nasor Valued Senior Member

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    Air resistance will slow whatever object you drop. Eventually it will reach a maximum speed and won't go any faster (called terminal velocity). A penny dropped off the sears tower would sting, but it wouldn't be like a bullet.
     
  9. Gifted World Wanderer Registered Senior Member

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    when comparing a penny and a brick, aerodynamics aren't big factors. They would reach the same velocity, but at different times. The brick would make a big hole, a penny a small one. The velocity would be enough to kill you, either way. Can't say anything about the water, though.
     
  10. James R Just this guy, you know? Staff Member

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    Does rain evaporate before it hits the ground? It falls from a lot higher up than the top of the Sears Tower.
     
  11. notPresidentAndrew Banned Banned

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    Good point, James.
     
  12. Nasor Valued Senior Member

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    This is wrong. A brick would get going much faster than a penny. I don't remember the equations for calculating terminal velocity off the top of my head, but large heavy objects tend to have much higher terminal velocities than small objects.
     
  13. chroot Crackpot killer Registered Senior Member

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    Force due to drag = force due to gravity.

    A bigger object has a lower suface area-to-volume ratio, which means it ends up reaching a higher terminal velocity.

    - Warren
     
  14. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    Terminal velocity is something like 9.8 metres per second per second, limited by the resistence met in falling.
     
  15. chroot Crackpot killer Registered Senior Member

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    No. 9.8 m/s<sup>2</sup> is the gravitational acceleration near the earth. It isn't even a velocity.

    Force due to drag = force due to gravity

    C<sub>d</sub> r V<sup>2</sup> A / 2 = m g

    V = sqrt ( (2 m g) / (C<sub>d</sub> r A) )

    - Warren
     
  16. Nasor Valued Senior Member

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    We can always count on chroot to whip out the equations that the rest of us don't want to bother with.

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  17. Gifted World Wanderer Registered Senior Member

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    A feather and a rock take the same time to fall in a vacuum(the expeirement was done on the moon, among other places). According to that, the only thing affecting terminal velocity in atmosphere would be drag. I'm not sure how mass would affect that.

    Equations are nice, when I understand them.

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  18. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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  19. chroot Crackpot killer Registered Senior Member

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    Terminal velocity is reached when the force due to gravity is equal to the force due to air drag. The force due to gravity involves the mass.

    - Warren
     
  20. Chagur .Seeker. Registered Senior Member

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    "... due to air drag."

    Wouldn't terminal velocity apply to drag in any fluid medium?

    Curious

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  21. chroot Crackpot killer Registered Senior Member

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    Generally, yes. Just change the value of 'r' in the equation I provided above to reflect the density of your fluid.

    In some cases, turbulent flow can arise around your object, and would make this simple calculation somewhat incorrect. It would probably still be within an order or magnitude.

    - Warren
     
  22. Rambler Senior Member Registered Senior Member

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    Turbulant flow will always develop around the object unless it's aerodynamically perfect (which is not possible as far as I know). Even extremely aerodynamic shapes will experience parasitic drag...i.e drag due to skin friction, and any point where the fluid sperates from the boundary layer will create eddies (drag)....as you can see to make the equations above perfect you would need to consider the behaviour of the flow around the object and factor in all of the drag components.

    The equations presented by chroot are an excellent approximation and infact at subsonic speeds are good enough to use for nealry all situations.
     
    Last edited: Jan 17, 2003

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