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Why do ships float?

  1. Oct 18, 2015 #1
    Does the amount of water displaced by an object needs to be greater than its weight (in air) for it to float in water?

    Say a bowl of 5 kg displaces 4 kg of water - will it float or sink? If it sinks the if it had displaced 5 kg of water would it float?

    That is the volume of water displaced by an object has to be greater or more than the weight of the object in air?
     
  2. jcsd
  3. Oct 18, 2015 #2

    Drakkith

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    Staff: Mentor

    It needs to displace an amount of water whose weight equals its own weight. If it displaces less, it will sink. If it displaces more, it will rise out of the water until it displaces just the right amount.

    It will sink. A floating object displaces exactly as much water as it itself weighs. That's why a ship fully loaded sits lower in the water than when it is empty. It has to displace more water to counteract the weight of its cargo.

    See the following links:

    https://en.wikipedia.org/wiki/Archimedes'_principle
    https://en.wikipedia.org/wiki/Buoyancy
     
  4. Oct 18, 2015 #3

    sophiecentaur

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    Those boats are very clever. They do those displacement / buoyancy / weight sums in their heads, without writing anything down and they always get it right.:biggrin:
     
  5. Oct 18, 2015 #4

    sophiecentaur

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    Well - there was the Titanic . . . .
     
  6. Oct 18, 2015 #5
    The amount of water displaced by a floating object is always equal to the objects weight. It never displaces more or less water as Sophie says the boat gets it just right.
    It really does not need to do anything except float.
     
  7. Oct 18, 2015 #6

    Drakkith

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    Note the distinction between a 'floating object' and an 'submerged object'. A submerged object usually doesn't displace an amount of water equal to its own weight. For example, rocks weigh much more than the water they displace, which is why they sink. On the other hand, the water displaced by a submerged beach ball weighs much more than the beach ball, which is why the ball will come shooting to the surface once you let it go.
     
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