• Question: how come when boats are made they dont sink as soon as they hit the water

    Asked by jamieb to Gioia, Iain, Jo, Leo, Mariam on 21 Jun 2010 in Categories: .
    • Photo: Iain Moal

      Iain Moal answered on 15 Jun 2010:

      Because boats work on Archimedes’ principle. Basically, the volume of water displaced by a floating object weighs the same as the object. So something really light, like a baloon, won’t dispace much water at all: we would say that it is boyant. Something that is heavier than water, like a rock, will sink to the bottom: we would say that it isn’t boyant. A boat, even if it is made of something heavier than water, like steel, will still float if it can dispace enough water without the water getting in.

      Take a look at this picture:

      The weight of the water that would be where the boat is if the boat wasn’t there, weights as much as the boat. If you were to add more weight to the boat, then the boat would sink and the sea-level would be further up the side of the boat. If you were to throw weight off the boat, then it would rise, and the sea-level would be further down the sides.

    • Photo: Mariam Orme

      Mariam Orme answered on 21 Jun 2010:

      I’ve forgotten a lot of my physics, but I think it’s because of their shape and because they are full of air. This website looks useful if you want to find out more:
      http://www.ypte.org.uk/environmental/boats-why-do-they-float-/107
      And this one has silly pictures but might be useful:
      http://boatsafe.com/kids/021598kidsques.htm

    • Photo: Leo Garcia

      Leo Garcia answered on 21 Jun 2010:

      Boats stay afloat because of the physics phenomenon of buoyancy. When a boat hits the water, it moves its own weight in water beneath it. The upward force acting on the boat is equal to the amount of that weight of water. In other words, a boat that displaces 50 kg of water will be pushed upward by 50 kg of weight by the water.

      So, if the boat displaces a weight of water more than the weight of the boat – it will float, because the upward force is greater than the downward force. Contrast this to a brick, say, where the weight of the displaced water is much less that the weight of the brick – it will sink.

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