• Question: How do things float?

    Asked by hannahjanehunt to Antonia, Douglas, Hugh, Matt, Tom on 22 Jun 2010 in Categories: .
    • Photo: Tom Hardy

      Tom Hardy answered on 21 Jun 2010:

      Because they are less dense than water!

    • Photo: Matthew Hurley

      Matthew Hurley answered on 21 Jun 2010:

      Floating happens because as the object goes into the water, some water is moved out of the way. The water that is displaced puts pressure on the object. If the force exerted by the water pressure is sufficient to keep the object up (like a piece of wood) the object floats, if the water pressure isn’t enough (like a brick) it sinks.

    • Photo: Douglas Blane

      Douglas Blane answered on 22 Jun 2010:

      Imagine you put something in water and push it right under. Say a wee yellow rubber duck.

      Now think about the duck-shaped lump of water that used to be in the place the duck’s in now.

      That lump of water was just sitting there minding its own business before. It wasn’t moving.

      So the weight of the duck-shaped water down must have been balanced by a push up. Because if you have two exactly-the-same forces pushing in opposite directions on something it won’t move. But if the forces are even slightly different it will move away from the bigger one.

      That push up that balances the weight of any lump of water that’s not moving comes from the rest of the water. It’s called the upthrust.

      So now back to the rubber duck. Because the upthrust comes from the rest of the water – which is still there – it pushes just as hard up on the duck as it did on the duck-shaped lump of water that used to be there.

      So if the duck is lighter than the duck-shaped lump of water the upthrust is more than the weight of the duck. So the duck will bob up to the surface and float.

      If the duck is heavier than the duck-shaped lump of water, its weight is more than the upthrust and the duck will sink.

      This whole thing is called Archimedes’ Principle after the ancient Greek who first realised how it worked.

      The story goes that Archimedes worked it out in his head while he was having a bath, and was so excited he jumped up and ran through the streets naked shouting “Eureka”. Which means “I’ve found it.” (And not “Oops I’ve no trousers on.”)

      The story doesn’t say if Archimedes was playing with a rubber duck in his bath. But I think he might have been 🙂

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