Calculating Total Force on Diving Bell's Port

[diva8789]

Homework Statement

An evacuated spherical diving bell containing a camera is in the ocean at a depth of 121 m. It has a flat, transparent, circular port with a diameter of 19.7 cm. Find the magnitude of the total force on the port (use ρseawater) = 1025 kg/m3).

Homework Equations

P(total)= ρgh+P(atm)
A=$$\pi$$R^2

The Attempt at a Solution

I know that you need to find the total pressure on the bell. And then i think you have to subtract that total pressure from atmospheric pressure. this is what i have been doing and it is not working correctly. does anyone have any ideas?

Thanx!

 

[Shooting Star]

Force = pressure x area. You know the pressure of the water at that depth, since density is known. Rest is easy. Atm presssure may be neglected.

 

[Dick]

Just to amplify what Shooting star is saying, the reason you can neglect atmospheric pressure is not because it is terribly small. It's about ten percent of the seawater pressure, but because you should probably assume there is also atmospheric pressure inside of the bell, so it cancels. Just use the rhoseawater they gave you together with the area.

 

[Bill Foster]

Total force on the port:

pressure/area from seawater on one side
pressure/area from 1atm on the other side

 

[Shooting Star]

Hi Dick,

The problem specifically states that the sphere has been evacuted. So, inside is almost vacuum.

To be realistic then, P_atm should be added to the water pressure, since 1/10 of the water pressure is not really negligible.

 

[Dick]

Hi Dick,

The problem specifically states that the sphere has been evacuted. So, inside is almost vacuum.

To be realistic then, P_atm should be added to the water pressure, since 1/10 of the water pressure is not really negligible.

In that case, yes, it should. Sorry, I should read more carefully!