Examkrackers Pressure Question

[dudup00p]

---

Members don't see this ad.

Two identical discs sit at the bottom of a 3 m pool of water whose surface is exposed to atmospheric pressure. The first disc acts as a plug to seal the drain as shown. The second disc covers a container containing nearly a perfect vacuum. If each disc has an area of 1meter squared, what is the approximate difference in the force necessary to open the containers? (note : 1 atm = 101,300 Pa)

Here's the picture:
http://imgur.com/xYO25

A. There is no difference.
B. 3000N
C. 101,300N
D. 104,300N

Here's the EK answer :

The only difference between the two discs is what they are covering. Ignore everything else. The first disc has atmospheric pressure pushing upward, the second disc does not. This is the difference in the forces necessary to lift them.

I don't understand how there's atmospheric pressure pushing upward when they're submerged in water. Doesn't the atmosphere exert pressure on the pool, which is incompressible and therefore spreads the pressure evenly and undiminished throughout the pool? Answer A just makes more sense to me...

Edit: Nevermind. I didn't notice that the first disc was covering a pipe which has atmospheric pressure pushing upward. Makes sense now.

 

[flashlightpen]

Sooo.... what is the answer exactly the books gives you? 🙂

 

[mehc012]

Ignore the fact that they're at the bottom of a pool. Picture 2 containers on the surface...one full of air, one with a vacuum. Which will be harder to open?
The vacuum will be. When it talks about 'atmospheric pressure pushing upward', it's really talking about the effect of the gas INSIDE the container. A gas is a collection of molecules with high velocity. These molecules bounce around the container, hitting the walls and bouncing off. This exerts outward pressure on the walls of the container (this is why a balloon inflates when you blow air into it). The vacuum container has no molecules on the inside hitting the lid and pushing it outward, and is therefore harder to open than the drain hole.

 

[Singhp03]

Sooo.... what is the answer exactly the books gives you? 🙂

answer has to be C:

(for the cover exposed to atm)
P=F/A
101,000=F/1
101,000N=F

(for the cover exposed to vacuum)
0=F/A
0=F/1
0N=F

thus, difference in force is of 101,000N or answer choice C

 

[cloak25]

answer has to be C:

(for the cover exposed to atm)
P=F/A
101,000=F/1
101,000N=F

(for the cover exposed to vacuum)
0=F/A
0=F/1
0N=F

thus, difference in force is of 101,000N or answer choice C

Hmmm..do you have to factor in the 3m depth?

 

[milski]

Hmmm..do you have to factor in the 3m depth?

No, the pressure due to the depth cancels when you calculate the difference between the pressures on the two lids.