Quick question on fluids and buoyancy

[testman2k]

Hi, i have a quick question regarding the above subject.

If

F(b) = (rho) (V) (9.8)

and F(b) = F(g)

and F(g) = (m) (9.8)

then wouldn't F(b) = (m) (9.8)

?

 

[FZ+]

A = B
B = C

Therefore, A = C

I don't see why there should be any problem...

 

[testman2k]

Then why do we need to use the formula:

F(b) = (rho) (V) (9.8) to find the buoyancy force when we could just multiply its mass by 9.8 ??

 

[FZ+]

Frog knows.

Really, what is the context of the question? If the object is floating - ie. weight = bouyancy, then obviously compute the weight and you get the bouyancy. Careful with V though - the equation will give you the submerged volume of the object, not the total volume of the object.

 

[testman2k]

the question is:

a submarine with volume(total) 4.67x10^3 and mass of 4.39x10^6 is floating at the surface of sea water, of density 1.03x10^3

a) what is the buoyant force on the submarine?

------------

In this case, we do not have enough information to use the equation F(b)=(rho)(V)(9.8)

since m = (rho)(v) - thus, (rho)=m/v

sub it in and you get

F(b) = (m/v)(v)(9.8)
simplify...
F(b)= (m)(9.8)

 

[testman2k]

another quick question when we plugin values for density for water do we use 1.0 or 1000

 

[FZ+]

Examine the units that you were given. The density of water is 1000 kg/m^3 and 1.0 kg/dm^3 or 1.0 g/cm^3.

If you keep the units consistent, you should be fine.

 

[testman2k]

What are we trying to match the units of density with?

 

[FZ+]

All your other units.