• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Need help answering problems related to Buoyance/Buoyance Force

  • Thread starter Sam
  • Start date

Sam

14
0
I have two homework problems that I am unable to answer related to buoyancy/force. Please help! Thanks!

1. How much force (in Newtons) does it take to hold a 100kg man completely under water in the ocean? His density is 903 kg/m^3, and the specific gravity of sea water is 1.07.

They give the following hint: Buoyant force is greater than the weight: B.F. = Weight + Force Down

2. What is the buoyant force in N on a 6.6 m^3 helium balloon in air at standard conditions? The density of air is 1.3 kg/m^3

They give the following hint: According to Archimedes you only need to know the weight of the air displaced.
 
512
0
Sam,
what have you got so far and where are you stuck? If you haven't started yet, then I think Archimedes' law is a good starting point.
 

Sam

14
0
Here's what I have so far...

I have the following, but I know my answer isn't correct because it's an online course and will let you know when you are correct:

Let:

Mm = mass of man in kg (given at 100 kg)
Vm = volume displaced by man in m^3 = Mm/Dm
Sg = specific gravity of seawater (given at 1.07)
Dw = Density of pure water 1 gm/cm^3 = 1000kg/m^3
Ds = Density of seawater
Fb = Buoyancy force
g = acceleration of gravity in m/s^2 = (9.8 m/s^2)

Here's how I tried to solve:

Fb = Vm*Dw*g
= (Mm/Dm)*Sg*Dw*g
= (100 kg/903kg/m^3)*1.07*(1000kg/m^3)*9.8m/s^2 N

Ok, so the buoyant force exceeds his weight. To keep him submerged you'd have to push him down with the difference force F=1161 - 980 or about 181 N.

What did I do wrong?
 
512
0
Re: Here's what I have so far...

Hmmm... looks OK to me. With the following changes, which don't change the result:

Fb = Vm*Sg*Dw*g
= (Mm/Dm)*Sg*Dw*g
= (100 kg/903kg/m^3)*1.07*(1000kg/m^3)*9.8m/s^2 no N

Since they say B.F. = Weight + Force Down, maybe they expect the answer to have a minus sign. Suggest you try that.
 

Related Threads for: Need help answering problems related to Buoyance/Buoyance Force

Replies
3
Views
750
Replies
2
Views
1K
Replies
10
Views
360
Replies
1
Views
1K
  • Posted
Replies
7
Views
3K
  • Posted
Replies
8
Views
10K
  • Posted
Replies
4
Views
6K
Replies
5
Views
12K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top