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Buoyancy and sinking objects

  1. Dec 8, 2015 #1
    When a dense object sinks in water, it sinks at a constant rate, because the buoyancy and fluid drag counteract the weight of the object.

    But if there is no resultant force, then why does the object slow down when it enters the water? Eg. if it's traveling at 2 m/s when it enters the water, shouldn't it continue to travel at 2m/s inside the water, because the forces are balanced?

    Is there some kind of force that slows down the object WHILE it's entering the water, but doesn't do anything once the object is fully in the water?
  2. jcsd
  3. Dec 8, 2015 #2


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    Science Advisor

    Why would you expect buoyancy and drag to be exactly equal to the weight of the dense object?
  4. Dec 8, 2015 #3


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    Staff: Mentor

    Hint: Drag is a function of speed.
  5. Dec 8, 2015 #4
    Okay so we did this experiment in school, and we dropped a steel ball into glycerin. After entering the water, it slowly sank at a constant speed, so that's how I know that the drag and buoyancy are equal to the weight.

    Does drag increase with speed? Is it directly proportional or something?
    That means the drag would initially be greater than the weight, but as the speed reduced, the drag would reduce until it's equal to the weight, and then the object would sink at constant speed.

    Also, is the buoyancy force stronger than the drag? Or is it the drag that mostly counteracts the weight?
  6. Dec 8, 2015 #5


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    It is not always as simple as a direct proportion. But yes, drag increases with speed.
    Bingo. The reverse happens as well. If an object is gently released then drag is initially zero and increases until the object sinks at constant speed. Google "terminal velocity".
    It depends on the situation. For a piece of waterlogged wood that is just barely more dense than water, buoyancy will be dominant and drag will be relatively small. For a lump of lead that is much more dense than water, drag will be dominant and buoyancy will be relatively small. For a ping pong ball flung into the water at high speed, drag will dominate -- briefly.
  7. Dec 8, 2015 #6
    Okay got it. Thanks
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