Random Questions

[Reginus]

A couple I thought of during science class:

Suppose you drilled a hole to the center of the moon (ignore the effects of heat, pressure, etc on a human being for now). You start climbing down a ladder. As you get closer to the center of the moon, does gravity get stronger, weaker, or does it stay the same?

Suppose the Earth stops rotating in relation to the universe. Do we get thrown against the wall, etc? Now suppose the universe stops rotating in relation to Earth, does this again happen?

Its kinda late at night, so I could very well wake up with a DUH moment when I think about these again.

[templeboy]

1) Well, the gravity is (much of a muchness) the same, but of course the deeper you go, the more it comes from different directions, so the net gravity once you reach the centre of the moon is 0 (except possibly some forces from Earth, the sun, Jupiter which might make the equilibrium position not quite at the centre, and might make it unstable. This would lead to a three-body situation, which is notoriously difficult. (http://en.wikipedia.org/wiki/Three-body_problem)
But the simple answer would be yes.

2) THe question doesn't really make sense, you need a reference frame to which you can say something is moving or rotating, and if theres anything that Einsteins taught us, it is that there is no default or simple universal reference to which we can compare our motion.

[iNow]

Suppose you drilled a hole to the center of the moon (ignore the effects of heat, pressure, etc on a human being for now). You start climbing down a ladder. As you get closer to the center of the moon, does gravity get stronger, weaker, or does it stay the same?

The force of gravity itself will actually stay the same, but the directions in which gravity is pulling you will change.  The strength of gravity is a function of mass.  The more mass there is, the stronger the gravity you will feel.

When you stand on the surface of a large massive object like the moon, all of the mass is below you, so all of the gravity you feel is pulling you downward.  However, as you start to go closer and closer to the center of the moon, then you have more and more mass above you.  You have some mass below you, and some mass above you, and they will cancel each other out a bit...  So, from your perspective, gravity will feel weaker since you're now being pulled up a bit, and also have less mass (hence gravity) below you... there's less moon there below you, so also less gravity... add that to the gravity pulling you upward from the mass now above you, and it would "feel" weaker.  

The strength of gravity per unit mass does not change, but what you perceive or feel DOES change since you are being pulled both upward and downward at the same time, and also since there is now less mass below you than there was at the surface.

Suppose the Earth stops rotating in relation to the universe. Do we get thrown against the wall, etc? Now suppose the universe stops rotating in relation to Earth, does this again happen?

Unfortunately, this question doesn't really make much sense since the earth is part of the universe... by definition.  However, let's just say that the earth stopped rotating and everything else carried on as per usual...  We'd still have the pull of gravity... we'd still have electromagnetism... we'd still have the weak and strong nuclear forces...  but the earth would not bulge at the center, and the liquid core would not slosh around in the same way, which would change the EM field around the earth quite a lot.  The weather would be all strange as wind patterns would be ... heck... I think wind would be non-existent, actually...  but you wouldn't be thrown against a wall or anything like that.

The reason you wouldn't be thrown against a wall is because you are part of the earth system.  Right now, the earth is spinning really fast, but so are we... right along with it.  If the earth stopped spinning, so would we, and we wouldn't really notice a change... at least, not relative to the earth.  

I sense that you are thinking about it like being in a car...  The car is going really fast, then you hit the brakes (or hit a wall or something), and you decelerate very quickly and you're thrown forward.  The reason this happens is because you are not fully attached to the system of the car.  The car has negative acceleration, but your adhesion to the seat is not strong enough to have your body decelerate at the same rate... The car stops, but you keep moving forward because your body is disconnected.

It's not really the same with the earth.  The mass of the earth is so large that the pull of gravity will overcome the slight "jerk" which would happen to us if it stopped spinning.  Further, you and the wall and everything are all stopping at the same rate, so the motion will be carried consistently through all objects... not just you who happens to be standing there.


Yeah... I'm pretty sure most of that is right.  Like you said, it's pretty late.  

[AlP]

Are you sure? I think it depends where you are. You'd be worst off standing on the equator. The planet makes one rotation per day (24 hours). The circumference is roughly 25,000 miles. So you're whizzing around the Earth at just over 1000 miles per hour. Imagine riding a skateboard down a road at 1000 miles per hour and it hits something and comes to an immediate stop. You'd continue to travel at 1000 miles per hour. In other other words, people on the equator would go flying if the planet suddenly stopped rotating. If you have reason to think it might happen (incoming asteroid maybe?), flee for the poles! =).

[Tanker]

first assuming the moon (or any planetary body) was perfectly spherial and the mass was equal throughout being in the exact center should nullify gravity ince the pull would be equal in all directions.

Second if watching Superman movies has tought me anything it's that reversing the Earth's rotation reverses time. So stopping the Earth's rotation should stop time. If time is stopped then nothing shold be moving and nothing would get thrown.

[AlP]

Second if watching Superman movies has tought me anything it's that reversing the Earth's rotation reverses time. So stopping the Earth's rotation should stop time. If time is stopped then nothing shold be moving and nothing would get thrown.

I thought you had to "slingshot around the sun"? Do star trek movies have more credibility than superman?

[iNow]

Are you sure?

Actually, on the second question, no... I was not.  Since it's impossible, and it breaks the laws of physics, then it's really difficult to use the laws of physics to describe what's happening.

However, it was an interesting thought experiment, and some googling tells me I was mistaken:


http://www.astronomycafe.net/qadir/q1168.html

If the Earth stopped spinning suddenly, the atmosphere would still be in motion with the Earth's original 1100 mile per hour rotation speed at the equator. All of the land masses would be scoured clean of anything not attached to bedrock. This means rocks, topsoil, trees, buildings, your pet dog, and so on, would be swept away into the atmosphere.

[AlP]

Oh don't worry. All I really said was speed is distance over time. If SSY sees this he'll kick our asses. Screwed too =).

[SSY]

I hadn't thought of the atmosphere, though I imagine even if the atmosphere were gone, you would still fly forwards with a  velocity of 1100mph from the equator.

Which raises an interesting question, If one is born at the poles, and, throughout their life, migrates down to the equator, they gain a load of rotational kinetic energy. Now, do you think you can measure this? Since the only source of this is the earth's rotation (which will be bled off to speed you up to 1100mph), which will connect to you through your feet, maybe there will be a small sideways force on you, through your feet, as you walk down to the equator? Is it possible to measure?  Or is this just the Coriolis force? Also, would you be lighter at the equator (according to scales of weight, not mass), since the "centrifugal force" force will be lifting you upwards? Do you think scale makers correct for this when selling scaled to people at the poles?

BRB, I have science to do.

Ok, at the equator, I weigh 3.84 newtons less than I weigh at the poles. I think this is significant, for a boxer or athlete that has to make a weight class, this could be the difference between passing and being disqualified.put another way, that is about 1/3 of one percent of my weight. This I imagine could be significant when making uop batches of chemicals, no good ordering chemicals from the equator, preweighed to sue at the pole, unless they were all weighed at the same longitude.