Acceleration in Water compared to Air

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Discussion Overview

The discussion revolves around calculating the velocity and kinetic energy of an object dropped from a height of 2 meters in water, compared to its fall in air. Participants explore the effects of different resistive forces in fluids, including buoyancy and drag, and how these factors influence the motion of the object.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that while gravitational pull remains constant, water significantly increases resistance compared to air, suggesting that the equations of motion are similar but must account for this added resistance.
  • Another participant argues that comparing acceleration in air without considering air resistance to that in water is misleading, as both are fluids and the modeling of resistance should be treated similarly.
  • A different viewpoint emphasizes the need to consider buoyancy, particularly for objects like a ping pong ball, which may float rather than sink.
  • One participant reiterates the initial question about calculating velocity and kinetic energy in water, suggesting that incorporating various forces such as viscous force and buoyancy vectorially is necessary for accurate calculations.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the problem, particularly regarding the treatment of resistive forces in air versus water. There is no consensus on the best method for calculating the effects of these forces.

Contextual Notes

Participants highlight that the resistance experienced by an object in a fluid is not constant and depends on factors such as velocity and the specific properties of the fluid. The discussion does not resolve the complexities of these interactions or the mathematical modeling required.

Rich M
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If an object is dropped in air through a relatively short known height (say 2m) on earth, then ignoring any air resistance its velocity on impacting the ground can be calculated from standard laws of motion. Its kineteic energy at impact can then be calculated if its mass is known. My question is what would be the approach for calculating the velocity and/or kinteic energy for the same object dropped through the same height in water?
 
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Hi there,

As a matter of fact, your object dropped into water will be subject to the same gravitational pull. The only difference is that water will resist the movement much more. But the equations are the same, with the added water resistance factor.

Ok for the simple theory behind it. Fact is that the resistance of air/water or any other medium cannot be considered constant, like we tend to do so in physics 101. The amount of friction developed by the surrounding material is dependent on the velocity of the object (dr/dt) and on a friction coefficient, which is not the same as \mu.
 
Comparing acceleration in air by "ignoring air resistance" to acceleration in water seems kinda like a cheat. Both air and water are fluids and the equations and approach one would use to model the resistance due to air or water are identical. However, in water these resistance effects are simply a lot more pronounced.
 
Need to add (subtact actually) buoyancy force. A ping pong ball will float.
 
Rich M said:
If an object is dropped in air through a relatively short known height (say 2m) on earth, then ignoring any air resistance its velocity on impacting the ground can be calculated from standard laws of motion. Its kineteic energy at impact can then be calculated if its mass is known. My question is what would be the approach for calculating the velocity and/or kinteic energy for the same object dropped through the same height in water?

Adding the forces like vicous force , upthurst force , Gravitational force and any other force if present vectorically will help youfind the velocity , then the kinetic energy
 

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