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How does buoyancy work?

Discussion in 'Electronic Design' started by Pimpom, Feb 18, 2013.

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  1. Pimpom

    Pimpom Guest

    Gravity and depth. The underside of the immersed body is at
    a greater depth than the upper side. Therefore the pressure
    on the lower part is greater than on the upper side, so
    there's an upward thrust. That's why the body feels lighter
    even if it has a density greater than that of the fluid.
    Working out the math from that should be simple for an
  2. mike

    mike Guest

    I know this isn't the right group for this question, but I want
    an answer an electrical engineer can understand.

    I understand the simple model of buoyancy. If something weighs
    less than the medium it displaces, it floats.

    The question is, "why?"
    If you stick a ping pong ball under water and let go, it floats.
    What's the mechanism? The water has no idea what's inside the
    ball. We have water molecules banging into the ball on all sides.

    Is there a simple explanation, in layman's terms, of what's actually
    going on to force the ball up?

    I really want to understand how convection works in a chimney,
    but water should be easier to grasp.
  3. Martin Brown

    Martin Brown Guest

    The simple answer is that the system is rearranging itself to obtain the
    lowest energy configuration possible. If the ball is too heavy this is
    with the ball at the bottom of the tank. Otherwise the ball will
    displace water and raise the water level in the tank against gravity
    until the net force it experiences is zero. That is once it has
    displaced its own weight of water it floats. Pushing it down or raising
    it from this equilibrium position increases its potential energy.

    The ping pong ball is accelerated by the vertical pressure difference
    across its diameter so if it is lighter than the water it displaces then
    upwards it goes. Various modern executive bubble toys exploit the
    behaviour of viscous oils and air bubbles a revamp of the lava lamps
    popular in the 1960's.
    Warm air rises because it is less dense and the heavier cold air falling
    can thereby lose some potential energy.
  4. Sylvia Else

    Sylvia Else Guest

    As you say, there are water molecules banging into the ball on all
    sides. But consider a horizontal slice of the water, one centimetre
    thick, at some arbitrary depth. It is being banged into by water
    molecules from above and from below, but it doesn't go anywhere, despite
    its weight, so its weight has to be balanced by a difference between the
    net forces generated by the molecules above and below it. The
    implication is that the rate at which molecules hit from below is
    greater than the rate at which they hit from above by an amount
    sufficient to create the required net force.

    This is just another way of saying the pressure rises as you go deeper
    into the water.

    So now if we go back to your immersed ping pong ball, it is apparent
    that the forces generated by the molecules banging on the ball are not
    balanced, because the forces on the lowerhalf of the ball are greater
    than the forces on the upper half. Thus there is a net upwards force on
    the ball.

    So how much net force is there? Well, if the pingpong ball were not
    there, then in its place would be a sphere of water. Since the water
    wouldn't go anywhere, the net force on that sphere must be equal to its
    weight. Thus the force on the pingpong ball is equal to the weight of
    the water that would have been there had the ping pong ball not been.

  5. SoothSayer

    SoothSayer Guest

    The reason why is gravity tugs one way ALL THE TIME. The molecules of
    one gaseous or liquid medium push against each other with a force based
    on their masses.

    So liquids or gasses introduced into this would rise or fall to
    whichever place their masses would put them with respect to the other
    members of the matrix.

    SOLID items, or solid items with a hollow inside and a gas of a
    particular pressure level filling that, will rise or fall to a particular
    place in the matrix based on what the mass of that exact same volume it
    displaces of the material it happens to be in at the time.

    So a one cubic foot displacing solid element in our sea level air has
    a zero point if it weighs the exact amount one cubic foot of sea level
    air weighs. If it weighs less, it will get pushed upward by the air
    around it (a balloon with Helium in it), until it reaches an equilibrium
    point that matches the weight it has against the matrix it is in (pretty
    high up).

    If it is in water, same rule applies. If that one cubic foot item
    weighs less than one cubic foot of sea surface, sea level water, it gets
    pushed UPWARD, and rides on said surface (in this case). as we cross
    into the realm where the one cubic foot device weighs more than the water
    it is in, it sinks (think submarine) toward the center of the earth, by
    gravity, and the fact that the molecules of water around it are pushing
    up less than said gravitational pull is pulling down.

    In free space, it merely changes the way said one foot cube would
    'slosh" through a given medium, based on relative velocities.
    as there would be no gravitational attractor in space. Down here, we
    merely get to examine how things move through slurries, and from that we
    can understand gravity. They found the Higgs Boson particle... maybe
    they'll see the gravity particle next.

    Ever see that surfer dude's multi-dimensional layout of the sub-atomic

    Anyway... gravity is the answer. and molecular pressure.
  6. SoothSayer

    SoothSayer Guest

    In other words, it only sinks a small bit because that small bit of
    volume, in the form of water, would weigh the same as the ping pong ball.

    So, if it displaces one teaspoon (for the sake of the analogy), then the
    weight of one teaspoon of water and one ping pong ball would balance on a
    scale (be the same).

    On the Moon, it would sink far less. And the water wouldn't last long
    either! (unless you were aboard The Eagle).
  7. You may want to think about that last statement a little longer....

    Jeroen Belleman
  8. SoothSayer

    SoothSayer Guest

    About the sinking, the water, or the Eagle?
  9. The sinking, specifically.

  10. mike

    mike Guest

    Thanks, guys, for the inputs.
    Those are the classical arguments. I accept them as true and useful
    models for everyday use.

    But, I'm still not satisfied.

    Throw a sheet of thin plywood into the swimming pool.
    If you put sufficient force on it, it sinks.
    Now turn it vertical and push it down.
    The force to submerge it should not change.
    But the direction of force applied by all the molecules banging
    on the side do not obviously have a net upward force.

    The thing I'm trying to get my brain around is convection.
    If I have a 20 foot chimney and build a fire in the firebox,
    air gets sucked in the box and goes up the chimney.
    If I cut a hole in the bottom and stack it on top of a
    similar chimney (close up the front), does the same amount
    of air get sucked into the bottom (cold) fireplace?
    My intuition says it does.
    And that means that if you're interested in ventilation,
    it doesn't matter where the heat gets into the pipe.
    Solar collector should work for that. And the distance
    to the space to be ventilated should be irrelevant???

    OK, so now, put an S-curve in the pipe, kinda like a P-trap.
    Can you make a passive thermostat out of it? Suck air
    only when the sun shines?
  11. rickman

    rickman Guest

    Uh, what? Well, no the vector of the force on the long sides now are
    opposing and so cancel out. But the unit area force on the bottom,
    small edge is much greater because it is at a greater depth. So the
    total force is the same. In the general case you have to integrate the
    forces over the entire surface. The rectangle of the plywood makes that
    easy. Do you know how to express this as an equation? It will be area
    (the bottom edge of your rectangle) times the unit area force
    (proportional to the depth).

    In the chimney case, as long as the temperature of the hot air being
    sucked in does not change a taller chimney will create a greater force
    because of the higher difference in air pressure from the top to the
    bottom just like in the water.

    I like Sylvia's way of looking at it. How much does the air in the
    chimney weigh vs. the air that would otherwise be in the chimney weigh.
    That is the total force pushing the air up the chimney.

    How fast it moves is another matter. A longer chimney will have more
    resistance but that may or may not be directly proportional to the
    length of the chimney. Air resistance is a complex subject.

    I'm not sure what you are thinking here. What would the S-curve in the
    pipe do? Are you putting water in it, or cold air, or what? Any sort
    of gas would just diffuse away I expect. Even a little water would
    create a huge barrier to air movement. That's how a water trap works on
    your sink. A water trap could work if it were shallow enough. But it
    wouldn't really act as a thermostat to maintain temperature in the room.
    It would just allow differences in pressure to be equalized with a
    minimum of air movement.
  12. Guest

    (snip to the real issue)
    OK, now think about why the air does that. Hint; buoyancy and
    convection are about *density*, not *mass*.

    Also, crack a window somewhere in the house or the chimney won't
    draw. Remember, this is a circuit and the fire is your potential
    source. ;>)
    IOW the chimney is now 40' tall. Fine.
    Intuition will usually get you into trouble in technical topics.
    What does your intellect say will be different with a longer chimney?
    Think resistors.
    Now you've built a fire on top of the chimney. My intuition mentions
    carts and horses.
    Wait, we've moved from heating (what fireplaces are usually designed
    for) to just moving air? How do you plan on coupling the heat from the
    solar collector to the chimney?
    An air version of a lambda diode? Interesting.

    Mark L. Fergerson
  13. Sylvia Else

    Sylvia Else Guest

    Convection works because, other things being equal, warm air is less
    dense than cold air. Since it's less dense, the pressure drop with
    height is less than the pressure drop for the same height in cold air.

    With a chimney, you then have the situation where the pressure at the
    bottom has to equal the pressure of the outside air at the bottom, and
    the pressure at the top has to equal the pressure of the outside air at
    the top, but the pressure change from the bottom to the top is
    different. The apparent impossibility is resolved by accelerating the air.

    The effect is thus dependent on the height of the column of warm air,
    and is consequently very dependent on where the heat is introduced.
    Well, if the convection is solar powered, then it will only occur when
    the sun is shining, regardless of the shape of the pipe. When it exists,
    the effect depends on the difference in altitude of the inlet and
    outlet. The only effect of an S-curve is to lengthen the pipe, and thus
    increase the frictional forces that slow the air down.

  14. For a "layman engineer," generic calculations involving integrating
    the hydrostatic pressure outside the submerged body should be easy to
    Analyzing simple shapes (cylinders, cubes etc.) can make it trivial.

  15. Laws of physics.....
    Leftists like to change Laws.

  16. Robert Baer

    Robert Baer Guest

    No, water will trickle out of the hand..
  17. SoothSayer

    SoothSayer Guest

    One might guess that the sinking would be exactly the same.

    I suppose it pretty much would be exactly the same sinking.

    But NO! The gravity is less, so it would *have* to sink less, and the
    rule includes that the water is standard Earth sea level density water.

    There is, however, no additional atmosphere to push down on it either.
    so, it would seemingly sink even less still. And the water would have to
    be lees dense than sea level Earth water. Hmmmmmm.

    Of course, it would also be boiling.

    Except, as I stated, within the pressurized confines of The Eagle.

    No air on top is about the only thing I see I could have missed
    including the effects of. But that should make my statement more true,
    not less.

    The ping pong ball would be bigger in a reduced pressure environ too,
    as it is filled with air.

    I don't get it. Don't see what I missed. You'll have to tell me.

    Nut floats on Mercury. What height does it float at here? On the
  18. H ow do you define 'pressure differential of the surrounding atmosphere'?

    Like starting an upside-down siphon, almost.
  19. SoothSayer

    SoothSayer Guest

    Again humor has been achieved.
  20. That's one of those laws of nature that seems to 'cause' things to happen as
    if they understand the law.
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