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I Do sinking objects lose momentum with depth?

  1. Mar 7, 2016 #1
    Hey guys, many might find my question stupid, but I could really really use an answer.
    Imagine a 1 cubic meter cylinder that weighs 1.2 metric tons. It displaces more water than its volume, so it will sink. For the sake of simplicity, let's imagine there's no drag and it's hydrodynamic.
    As it reaches the depths of an ocean, does it slow down in its descend due to the increasing water pressure? And if so, how does one calculate that?
    Also, let's pretend that the bottom of the ocean is at 100m. If I put a scale down there and the cylinder comes to a stop on top of the scale, what weight will the scale show me?
  2. jcsd
  3. Mar 7, 2016 #2


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    Hello Adrian, :welcome:
    How does it do that :smile: ?
    Well, if there is no drag, the only forces are from gravity and from buoyancy. The resultant is a constant (*), so according to F = ma it will not slow down but keep accelerating uniformly ...
    I gave that away, didn't I ? Except I didn't specify the buoyancy force. Can you help me with that ? Archimedes or something ?

    And I kind of also gave away that it doesn't matter if the floor is at 50, 100 or 500 m: the scale will show this constant force....

    (*) for simplicity I let the density of water be independent of the pressure.
  4. Mar 7, 2016 #3

    Thank you so much for your answer!
    I did mean to say that it weighs more than the volume of water it displaces and so it sinks.. but somehow I got confused.
    As for the other things, well I do appreciate sarcasm if it makes me realize where I did wrong.

    You guys keep up the good job! Been looking for an answer to my question for 2 days and I got it from here in 45 minutes.
    Last edited by a moderator: Mar 7, 2016
  5. Mar 7, 2016 #4


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    So do you see how this part of your question isn't physical? All the rest is fine, but if you want to understand the physical situation, you need to be realistic.

    As BvU alluded to, without drag, the block will keep accelerating without bound. That's not physical, right?

    With drag, the block reaches its terminal velocity fairly quickly, and keeps sinking at the same terminal velocity rate independent of depth (contingent on the * note by BvU). Does that make sense? And the underwater weight of the block will be constant with depth, again, as long as the water is incompressible (which it mostly is). :smile:
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