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Can something more heavy than water float

  1. Dec 2, 2010 #1
    hi all i just read that surface tension floats a ferry boat like the same way a pin can be placed very carefully on the surface of water, is this true, i always thought that if the boat displaced less water than it could hold it would float, i know there is huge holes in my logic but bare with me im just a metal fabricator with lots of questions ,so its safe to say that if i put more weight than the weight of water the boat displaces it will sink ,thats logical isnt it providing the boat is made of something the same weight of the water it is floating in....... can someone tell me in laymans terms what is e=mc2 because im realy interested in physics but im having trouble understanding this and i know its a biggie, thanks in advance,john
  2. jcsd
  3. Dec 2, 2010 #2


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    hi gttjohn! :smile:
    no, the surface tension effect is very small, and nowhere near enough to float a boat …

    as you say, a boat whose hull is heavier than water floats because it can displace its weight of water (because its average density is lower than water)

    perhaps they were referring to a water boatman? … see http://en.wikipedia.org/wiki/Corixidae" [Broken]
    e is the energy in, say, uranium

    m is the mass of the uranium

    c is a conversion factor (like the conversion factor between joules and electron volts) … it happens to be the square of the speed of light (but light itself has nothing to do with the equation)

    e = mc2 is the equation that tells you how much energy you could get out of the uranium if its mass was completely destroyed (it can't be, of course … you can only get it down as far as lead … though you can completely destroy the mass of an electron and a positron, and the same equation applies) :smile:
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  4. Dec 2, 2010 #3


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    For what it's worth, E=MC^2 is relatively unimportant as you won't need to use or understand it until much later in your physics career. More important in my opinion for a physics beginner is F=m*a, the http://en.wikipedia.org/wiki/Kinematics" [Broken].

    Of course, that's from a mechanical point of view...
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  5. Dec 2, 2010 #4
    thank you very much its slowly becoming clearer ive just gotton the grasp of the doplar effect red shift and blackholes,this forum is amazing ,,,doplar now explains why the pitch on an ambulance changes as it comes to you and then passes ,thanks again i have loads of questions
    im off to bed now thanks again
  6. Dec 3, 2010 #5
  7. Dec 4, 2010 #6
  8. Dec 5, 2010 #7
    Archimedes figured out that the buoyant force. It is the force that makes things float and also makes things feel lighter in a fluid (like water). Basically, the upward force is equal to the weight of the displaced volume (mathematically, Fb = density*acceleration due to gravity*displaced volume). This applies whenever one fluid is displacing another (and there is gravity).

    A boat floats even though the metal it is made of sinks. That is because the boat is hollow, which means a large part of the volume is filled with air, which is much less dense then the water. The Average density of the boat is less then the average density of the fluid so it floats. The relative difference between the density difference determines how much of the boat is submerged. This is also how hot air balloons and blimps fly.
  9. Dec 5, 2010 #8
    this just opens up another question why does something lighter than air rise up like what is the mechanics behind it
  10. Dec 5, 2010 #9
  11. Dec 6, 2010 #10


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    hi gttjohn! :wink:

    you may think that air https://www.physicsforums.com/library.php?do=view_item&itemid=80" is the same all the way round something, but in fact there's a slight difference the higher you go (that's how an aeroplane's altimeter works) …

    even with something as small as a child's balloon, there's less pressure (and therefore downward force) on the top of the balloon than on the bottom, and so the net force is upward …

    the difference in pressure is ∫ρairgA(h)dh = ρairVg = mairg = Wair,

    where A and V are area and volume, and ρair and mair and Wair are the density of air and the total mass and total weight of the air displaced

    so applying good ol' https://www.physicsforums.com/library.php?do=view_item&itemid=26" to the balloon,

    ma = W - Wair = mg - mairg

    = Vgρ - Vgρair (where ρ is the average density of the balloon)

    = Vg(ρ - ρair) = mg(1 - ρair/ρ) …

    ie the "effective weight" of the balloon is multiplied by a factor (1 - ρair/ρ) …

    if ρ > ρair, the effective weight is reduced slightly, if ρ = ρair, the effective weight is zero, and if ρ < ρair, the effective weight is negative, and the balloon rises! :wink:
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  12. Dec 7, 2010 #11
    thanks tiny-tim but if i hold a balloon at sea level wouldnt there be miles and miles of air on top of it ,has it got something got to do with the air particles rushing in under the lighter balloon thus forcing it up ,thanks for your time
  13. Dec 7, 2010 #12


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    yes, there's miles and miles of air on top,

    but there's miles and miles of air plus a few inches on the bottom …

    it's those extra few inches that make all the difference!

    there's more force underneath pushing up than there is above pushing down …

    a total net force up

    if that net force is more than the weight of the balloon, then the balloon rises :smile:

    (applies to golf balls and everything else of course … but golf balls are so heavy, compared with air, that we don't notice the difference :wink:)
  14. Dec 7, 2010 #13
    aaah i see ,thanks again
  15. Dec 13, 2010 #14
    The guys have been doing you proud on this one. Just a few points.
    The reason a pin or a razor blade (does one still get them?) will float on water is that it is too light to break through the apparent "film" of the water's surface tension. As it gets wet, it slowly breaks through and when enough of it has got through, it will sink pretty smartly.
    Even at that, while it is floating, it is not the surface tension holding it up, but the buoyancy of the metal (which does have some buoyancy, just not enough to float by itself; it weighs less on a spring balance underwater, than in air) plus the air in the space above the metal and lower than the water surface held back by the surface tension.
    That space is very shallow. If you try to make it deeper by pressing the metal down, the greater water pressure overcomes the surface tension, and your pin or blade sinks pronto.
    If you put a heavier object, say a nail, on the surface tension, it also pushed too far down and the surface tension fails at once and the nail sinks.
    Now, think of the ferry boat. Sure it floats for partly the same reason, namely that the water can't get in from the side, but instead of a teensy-weensy "film" of surface tension, we cheat and give it massive steel bulwarks that keep the water back, even though the surface tension failed at the first water contact when we launched the ferry. If we seal the ferry and push it underwater it will pop up again, but if we push it a couple of kilometres down, its walls would fail just as the surface tension did, and it would sink the rest of the way.
    I hope that helps a bit with parts of the idea. Quite seriously, you might help get it clear in your mind by playing with surface tension. See what sorts of thing will float and for how long. If you have a glassful of clean water in a clean glass and you float a bit of foam plastic on it, does it float near the side or the centre. If you start dropping in more water till the water surface is level with the top or even starts to bulge over, what happens?
    There is a marvellous book called "Soap Bubbles" by C. V. Boys. You might find it well worth a read. It is online at:
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