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

[tennisgirl92]

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

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

The Attempt at a Solution

F_b=(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?

 

[kuruman]

Fb=(1.2 kg/m³)x (9.8) (.0854)

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.

 

[FactChecker]

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?

 

[tennisgirl92]

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.

 

[tennisgirl92]

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.

 

[FactChecker]

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?

 

[tennisgirl92]

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.

 

[gneill]

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.