What factors affect the rate of sinking objects in water?

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Discussion Overview

The discussion revolves around the factors affecting the rate at which objects sink in water, specifically examining three instances involving blocks of varying mass and size. The scope includes theoretical reasoning based on principles of physics, such as Newton's second law and buoyancy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant proposes that in the first instance, heavier blocks fall quicker due to increased acceleration as mass increases, while the weight of water remains constant.
  • In the second instance, the same participant suggests that smaller blocks fall quicker because, although they have the same mass, the weight of water increases with size, affecting acceleration.
  • For the third instance, the same participant argues that all blocks fall at the same rate since the densities are equal and the volume cancels out in the equations.
  • Another participant agrees with the reasoning presented for instances one and two, and also concurs with the conclusion for instance three, noting that it is straightforward to express acceleration mathematically.
  • A different participant emphasizes that density is a key factor, stating that objects with greater densities sink faster than those with smaller densities.
  • Another participant references Galileo's experiments, suggesting that the discussion relates to historical physics concepts and acknowledges the complexity of the topic.

Areas of Agreement / Disagreement

Participants generally agree on the reasoning for instances one and two, while also agreeing that all blocks in instance three fall at the same rate. However, there is no explicit consensus on the implications of density across all instances, leaving some aspects of the discussion open to interpretation.

Contextual Notes

The discussion includes varying interpretations of the effects of mass and size on sinking rates, and the participants express some uncertainty regarding the clarity of the equations presented. The implications of density and buoyancy are acknowledged but not fully resolved.

jrwints
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Hi All,

I have a problem (with 3 separate instances) to which I believe I have the answers, but would like check with those more knowledgeable than myself. They revolve around 3 blocks sinking through water and which falls quicker. I am ignoring friction.

Instance 1:
All blocks are exactly the same shape and size. They have different masses however. Which falls quicker?

Instance 2:
All blocks are of different sizes (short/medium/long), but have the same mass. Which falls quicker?

Instance 3:
All blocks are made of the same material but are different sizes (short/medium/long). Which falls quicker?

My understanding of the answers is based upon Newton's second law, buoyancy and free body force diagrams:
W(obj) - W(wat) = mass of object x acceleration (where Weight of water is equal to its density x volume of object x gravity):

a- ( m(o) - m(w) ) g = m(o) a
b- ( p(o) - p(w) ) vg = p(o) v a (mass = density x volume)

1. Using eqn a: Although m(o) increases, m(w) remains constant, therefore acceleration increases and heavier mass falls quicker

2. Using eqn a: As they have the same mass, m(o) remains constant, but m(w) increases as the size of the object increases. Therefore, the smaller object falls quicker

3. Using eqn b: All densities are equal, but as the volume on both sides of the equation cancel each other out, they fall at the same rate

Is this the correct understanding? I believe it is, but it does do your head in!
 
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Hello jr, :welcome:

If you want to ignore friction, you are back to Galileo Galilei's experiments.
You equations are hard to read and understand. are the a and the b enumerators or variables ?

[edit] all right, enumerators.

Instance 1 and 2: I agree. Some can even move upwards...
Instance 3: agree also.

In all cases it's pretty straightforward to write an expression for the acceleration.
 
Thanks BvU,
It basically comes down to density. Those with greater densities sink faster than those with smaller densities.
It's basic physics but something I haven't needed to think about or do for ages - when they come up in aptitude tests you forget the equations and logic behind the reasoning!
 
Same here: Galilei had to do with non-vacuum and was genius enough to reason through that limitation. For a fish such luck isn't there :smile:
 

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