Weighing Floating Cork: Does Upthrust Affect Weight?

  • Context: High School 
  • Thread starter Thread starter rishch
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the question of whether the weight measured by a scale changes when a floating cork is placed in a beaker of water. Participants explore the implications of buoyancy and forces acting on the system, considering both theoretical and conceptual aspects of weight measurement in this context.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the total weight measured by the scale would be the sum of the weight of the beaker, water, and cork (x+y).
  • Others argue that the weight would not be affected by the cork floating, suggesting that the system remains in equilibrium and the scale must exert a force equal to the total weight of the system.
  • A participant introduces the concept of buoyancy, stating that the upward buoyant force on the cork is equal to its weight, and discusses how this force interacts with the water and the scale.
  • Another participant questions how the force of gravity acting on the cork is transferred to the scale, prompting a comparison to a scenario involving a person standing on a scale with a book on their head.
  • Responses clarify that the downward force from the cork on the water contributes to the total force the scale must support, similar to the book analogy.

Areas of Agreement / Disagreement

Participants express differing views on whether the weight measured by the scale changes with the addition of the cork. Some believe it does not change, while others support the idea that the total weight is the sum of all components. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Participants rely on assumptions about equilibrium and forces without fully resolving the implications of buoyancy and weight transfer in this specific scenario.

rishch
Messages
106
Reaction score
1
Suppose you have a beaker containing some water and you weigh it using a weighing scale (the ones you use to measure your own weight ) and its weight is x.You also have a cork which has a weight y.Suppose you put the cork in water and it floats and you then place the beaker with the cork floating in it on the weighing scale.Would its weight be x+y or would the upthrust on the cork affect the weight of the entire system?
 
  • Like
Likes   Reactions: EH2011
Physics news on Phys.org
rishch said:
Would its weight be x+y or would the upthrust on the cork affect the weight of the entire system?
What do you think?
 
I think that the weight wouldn't be affected.
 
rishch said:
I think that the weight wouldn't be affected.
So you think you can support beaker + water + cork with the same force as you do just beaker + water?

Since the system of beaker + water + cork is in equilibrium, the net force on it must be zero. Gravity pulls down with the full weight of all the mass; so the scale must push up with an equal force.
 
Thanks !
 
Another way to think of it:

Since the cork is floating, there must be an upward buoyant from the water acting on the cork that is exactly equal to the cork's weight. But by Newton's 3rd law, if the water pushes the cork up, the cork must push the water down with an equal and opposite force. That downward force on the water adds to the upward force that the scale must exert to support the water.
 
Since the cork isn't actually in contact with the weighing scale then how does the force of gravity acting on the cork get transferred to the scale?
 
rishch said:
Since the cork isn't actually in contact with the weighing scale then how does the force of gravity acting on the cork get transferred to the scale?
In the same manner as this: Imagine you were standing on a scale. It reads your normal weight. Now someone balances a book on your head. Now the scale reads your weight plus the weight of the book. The book is not in direct contact with the scale, so how does the scale 'know about' the book on your head?

The book pushes down on your head with a force equal to its weight. Thus, to support you the scale now has to exert a greater force than just your weight, since it must counter balance the added force from the book on you as well as your weight.

With the cork and water it's the same idea. The cork pushes down on the water just like the book pushes down on your head.

Make sense?
 
So the weight of the book gets transferred to the scale through you?
 
  • #10
rishch said:
So the weight of the book gets transferred to the scale through you?
Sure. No magic here!
 

Similar threads

  • · Replies 19 ·
Replies
19
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 10 ·
Replies
10
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 23 ·
Replies
23
Views
3K
  • · Replies 28 ·
Replies
28
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 16 ·
Replies
16
Views
7K