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Pressure and buoyancy

  • Thread starter A_lilah
  • Start date
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1. Homework Statement
An iron anchor of density 7870 kg/m3 appears 290 N lighter in water than in air.
What is the volume of the anchor?



2. Homework Equations
Fnet = 0 (we are doing static fluids) = forces up - forces down

Fbuoyant = density of liquid*g*volume of object

density = mass/ volume, so mass = density*volume

Weight in the air = mg, so weight in the water = (mg - 290N)

density of water = 1000 kg/m^3

3. The Attempt at a Solution

Fnet = Fbuoyant - (mg - 290N)
0 = 1000 * g * volume - (7870* 9.8*volume - 290N)
-290N = 9800 * volume - 77126 * volume
factor out volume, and

-290/ -67326 = volume = .00431 m^3

(the units work out right, I just left them out here to try to make things clearer)
unfortunately this answer is wrong....
any help would be great!
thanks
 

Answers and Replies

1
0
It's simpler than it looks.

All you need is the Fbouyant equation, with 290 N on the left hand side and the values for g and the density of water on the right. Then solve for V. (This assumes that you can neglect the density of air (=1.29 kg/m**3) in comparison to the density of water, and, since the bouyant force is only given to 3 figures, this seems to be the case.)
 

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