# Homework Help: Boyancy Problem

1. Apr 7, 2005

What in am doing wrong? I assumed it's just the volume of the tank and then you find the weight of the water displaced. It's not working. I made sure that the diameter was changed into a radius for the volume of the sphere. Also as a sanity note, if you have a tank that sinks when it's empty it should still sink if it's full of air right?

Last edited: Apr 7, 2005
2. Apr 7, 2005

### dextercioby

It's really simple.
a)[itex] F_ {APXIMN\Delta N\Sigma} = V_{sphere}\rho_{water} g [/tex]

That's the scalar.

For point b),apply Newton's second law for the sphere.How many forces act on the batiscaph?

Daniel.

3. Apr 7, 2005

Yeah which is why I hate it when I get these problems wrong.:) I know that has to be the answer. I can't figure out why Im not getting the right number. Here is what I am getting. 1108353.888 Newtons.

4. Apr 7, 2005

### ramollari

Dex why are you writing hieroglyphs for very simple formulas .

Adam, $$F_B = \frac{4}{3}\pi r^3\rho_{w}g = F_B = \frac{4}{3}\pi (6m)^3(1000kg/m^3)(9.8m/s^2) = 8.87 * 10^6 N$$.
Right?

5. Apr 7, 2005

Yeah. Your equation is right. Though don't you need to divide the diamter by two to get the radius? I checked neither of those two possible solutions are correct.

Last edited: Apr 7, 2005
6. Apr 7, 2005

### QuantumMech

The problem I did like this used

$$\begin{multline*} \Sigma F_{y}=F_{b}-F_{t}-F_{g}\\F_{g}=ma \end{multline*}$$

with gravity and tension in the negative y direction.

Last edited: Apr 7, 2005
7. Apr 7, 2005