The Buoyant Force on a Man: What is the Ratio to His Weight?

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Homework Help Overview

The discussion revolves around a physics problem involving buoyant force, weight, and density. The original poster presents a scenario where a man has a specific mass and density, and seeks to calculate his volume, the buoyant force exerted by air, and the ratio of this buoyant force to his weight.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the calculation of buoyant force using the density of air and question the correctness of the density values used. There are attempts to clarify the relationship between buoyant force and weight, and participants express uncertainty about the impact of rounding on their results.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and questioning the assumptions made regarding density and constants. Some guidance has been offered regarding the significance of using accurate values for constants, but no consensus has been reached on the correct approach or answer.

Contextual Notes

Participants note that the problem is part of a web-based homework assignment, and there is a lack of access to the correct answer, which may be influencing the discussion. There is also mention of significant figures in the constants used for calculations.

tennisgirl92
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Homework Statement


A certain man has a mass of 85.0 kg and a density of 995 kg/m3 (exclusive of the air in his lungs).
(a) Calculate his volume.
.0854 m3
(b) Find the buoyant force air exerts on him.
N
(c) What is the ratio of the buoyant force to his weight?
(buoyant force / weight)

Homework Equations


Fb=density (of substance) x Volume (of object) x g
density of air=1.2 kg/m3

The Attempt at a Solution



Fb=(1.2 kg/m3) x (9.8) (.0854)
=1.004N

and then for part c
1.004/ (9.8 x 85=833)=.00121

For some reason this is incorrect. Does anyone see why?
 
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tennisgirl92 said:
Fb=(1.2 kg/m3)x (9.8) (.0854)
tennisgirl92 said:
A certain man has a mass of 85.0 kg and a density of 995 kg/m3
Incorrect density.

Edit: Sorry I misread the question.
 
Last edited:
For the buoyant force of the displaced air, the air density should be used as the OP shows. How far off is the answer? Could it be just due to rounding of 1.225 to 1.2?
 
Yes, I tried that as well and obtain 833N, which is also incorrect. I did this first, and then thought we need to use the density of the air.
 
FactChecker said:
For the buoyant force of the displaced air, the air density should be used as the OP shows. How far off is the answer? Could it be just due to rounding of 1.225 to 1.2?

yes, I tried 1.225 and 1.2. Both are incorrect.
 
tennisgirl92 said:
yes, I tried 1.225 and 1.2. Both are incorrect.
Just so we don't have to keep guessing, what is the answer given?
 
FactChecker said:
Just so we don't have to keep guessing, what is the answer given?

I don't have access to the answer yet-it's a web assign HW question.
 
Try to make sure that all constants used have as at least as many significant figures as the given data. That includes your value for g.
 
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