Pop quiz: diving on the moon

[Zept]

MikeS... there would not be enough atmospheric pressure for water to stay in a liquid state

Bother, good point, I missed that one. I should have sent them to an imaginery planet with more suitable physics.

Newhampster... If you used an SPG (pressure gauge), it would automatically compensate for the change in gravity so...when it read 33 feet, you'd really be at a measured 198 feet but the pressure would be 14.7 psi...

I think you mean depth gauge, not SPG... but you are right, a depth gauge is basically a pressure gauge, and it is the pressure that matters, not the distance from the surface, when you're thinking about nitrogen absorption -- as far as I can see, anyway. Anyone want to correct me?

Rick... Our Apollo astronauts used full pressure suits pressurized to 5psi, breathing pure oxygen.

Points to Rick for spotting the (deliberate) trap: you not only have to know what the astronauts have in their tanks, but also what they are breathing in the spacecraft, and at what pressure. Rick's assumption of pure oxygen in the tanks is very reasonable, but suppose they'd absentmindedly filled them with air? Very nasty if they forget that the pressure in their spacecraft is only 0.35 ATMe.

Uncle Pug... I am proud of you.... nice hijack....

Sorry, I wasn't trying to steal your audience . My problem was inspired by your problem, but it didn't seem entirely relevant to that thread .

Someone else's turn now...


Cheers all, Zept

 

[Uncle Pug]

Originally posted by Zept
Sorry, I wasn't trying to steal your audience . My problem was inspired by your problem, but it didn't seem entirely relevant to that thread .

Noooo... Zept.... I meant that... it's a tactic I have used on occassion and your's was very smoooooth... a little spacey perhaps... but I'm still proud of you!

 

[donacheson]

Originally posted by MikeS
Even ignoring the temperature issue, with one-sixth earth’s gravity there would not be enough atmospheric pressure for water to stay in a liquid state. Water must be pressurized to stay in a liquid state; otherwise it will turn to a gas. The 14.7 psi at earth sea level is more than sufficient.

Sorry to be a party pooper but if you change the laws of physics to let water exist without pressure, the tables, based on the same laws of physics would no longer be valid.

Mike

Not so. The physics requirement is that the liquid water be in equilibrium with its vapor above it, not the total pressure. The original question didn't specify the composition of the atmosphere on the moon nor the water temperature, so one can't determine atmospheric pressure. If the atmosphere consisted only of water vapor, atmospheric pressure would range from 0.088 psia at 32F to 0.61 psia at 86F.

 

[MikeS]

Don,

The gravity of the moon is not sufficient to “hold” a water atmosphere, the water vapor would vent into space. Additionally, the water vapor would migrate to the dark side of the moon where it would condensate and freeze to form ice. As venting and freezing reduced the pressure, more liquid would transition to a gas until it was all gone.

Something else to consider before diving on the moon, watch out for the tides! The tides on earth are caused by the gravitational pull of the moon. In the moon’s case the tides would be caused by earth’s much greater gravity. Although high tide would only occur once a month, they would be impressive!

Mike

 

[Blargh]

Originally posted by Zept


Bother, good point, I missed that one. I should have sent them to an imaginery planet with more suitable physics.

<snip>

Cheers all, Zept

Try Europa, one of Jupiter's moons. Liquid water and no atmosphere. One would need to cut a hole in the ice first ......

Dont ask me about its gravity though

 

[Newhampster]

Something else to consider before diving on the moon, watch out for the tides! The tides on earth are caused by the gravitational pull of the moon. In the moon’s case the tides would be caused by earth’s much greater gravity. Although high tide would only occur once a month, they would be impressive!

Huh? Why would there be tides on the moon? The gravitational force wouldn't vary across the month. Since the moon rotates on its own axis once a month, and that's synchronous with the behavior of its orbit, the moon always presents exactly the same face to the earth -- so the effect of the earth's gravity on the moon is constant.

QED -- no tides on the moon.

Did I forget something? [Probably...please correct this]

 

[jbm]

Great discussion but all moot. It is a historical fact that the preferred recreational activity of lunar visitors is golf and the occasional off-road 4 wheeling.





jbm

 

[MikeS]

There would be no tides on the moon if it had a circular orbit, however it is elliptical. The difference in the distance and hence the gravitational pull is why the high tide level for a given spot on the earth changes during a month. This would cause tides on the moon.

 

[Zept]

In the quest for better science, I have changed the scenario:

-----
Two space tourists land on the moon and check into the Lunar Holiday Inn [substitute hotel of your choice]. The hotel's amenities include a large, deep, ornamental lake of pure water. The lake is covered by a dome that keeps out solar radiation, micrometeorites and other space nasties, and keeps in a breathable atmosphere at a reasonable temperature.

The tourists are keen divers (of course) and want to dive in the lake. But, hmm...

Can they use their dive computers?

Can they use their dive tables?
-----

Same problem, so the answer(s) should be the same, but I think it's more feasible from a physics point of view. Designing and building the dome might require a certain amount of engineering ingenuity, but that's a problem for the future.

Any comments? Ideas for further improvements?

I'm still not convinced that a body of water on the 'face' of the moon (the area directly opposite Earth) would experience tides. The gravitational force exerted on it by the Earth would change over the month, but the force would always be acting in the same direction -- directly upwards. On Earth we experience tides because the direction of the force keeps changing, relative to the position of any particular coast.

Z

 

[UKPhil]

It's one small step for man,
one giant stride for mankind