Does an object slow down when it enters water?

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    Buoyancy
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Discussion Overview

The discussion revolves around the behavior of dense objects when they enter water, specifically addressing the forces at play such as buoyancy and drag, and why objects appear to slow down upon entry. The scope includes conceptual understanding and experimental observations related to fluid dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that when a dense object sinks in water, it reaches a constant sinking rate due to the balance of buoyancy and drag against its weight.
  • One participant questions the expectation that buoyancy and drag would equal the weight of the object at all times.
  • It is noted that drag is a function of speed, which influences the behavior of the object as it enters the water.
  • Experimental observations are shared, indicating that a steel ball dropped into glycerin sinks at a constant speed, suggesting that drag and buoyancy equal the weight at that speed.
  • Participants discuss whether drag increases with speed and the nature of its relationship, indicating that it is not always a direct proportion.
  • There is a suggestion that initially, drag may be greater than the weight of the object, but as speed decreases, drag also decreases until it equals the weight, allowing for constant sinking speed.
  • Participants explore the relative strengths of buoyancy and drag, noting that this can vary depending on the object's density and the situation.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the dynamics of drag and buoyancy, and the discussion remains unresolved on several points, particularly concerning the initial forces acting on the object and the conditions under which they balance.

Contextual Notes

Limitations include assumptions about the conditions of the experiment, the definitions of drag and buoyancy, and the specific scenarios being discussed, which may not apply universally.

Nishantkumar19
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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?
 
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Nishantkumar19 said:
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.
Why would you expect buoyancy and drag to be exactly equal to the weight of the dense object?
 
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?
 
Nishantkumar19 said:
Does drag increase with speed? Is it directly proportional or something?
It is not always as simple as a direct proportion. But yes, drag increases with speed.
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.
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".
Also, is the buoyancy force stronger than the drag? Or is it the drag that mostly counteracts the weight?
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.
 
Okay got it. Thanks
 

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