Question about gravity and mass

[NHFoxtro]

The way I get it. Because its moving side ways at almost 3 times the speed its falling.The two speeds cause it to orbit in the same position, instead of moving closer or falling.

Its been a long time since I read anything on the solar system. Basicly I'm winging it. Don't even know if I'm close to the rite answer.  

[Ripsnort]

Er, guys, the moon is moving toward earth, and one day, will crash into it,..if memory serves me correct, I think the Discovery channel said something like 10.2 billion years, or something astronomical number...

Correction, see post below...thats the Earth moving towards the Sun?

[This message has been edited by Ripsnort (edited 01-29-2001).]

[sling322]

Ummm...Off topic?  Hee Hee....just kidding HT!!  

[mrfish]

the moon is moving away from the earth - not toward .....about 4 cm per year

it'll take about 15 x 10 9th years - we will be toast in about 1/2 that time anyway and even if we were around the orbit would steady out at about 2x the present distance

[Ripsnort]

Thank you, AWAY, not towards...man, the mind is the first thing to go, and the 2nd is...hmm, I forget...

 

As far as the orbit decaying, actually currently due to the tidal
                   forces between the Earth and moon the distance between is
                   increasing. The spin of the Earth is literaly pulling the moon faster
                   which causes it to orbit farther away. It will be far into the future
                   before the moon tidal locks the Earth and starts to decay. We'll have
                   to worry about being burned up by the sun first!

[This message has been edited by Ripsnort (edited 01-29-2001).]

[Tac]

Cc
 

The moon will eventually escape earth's gravity. As it moves away, the earth's gravity pulls it back in which in turn makes the moon's gravity pull the ocean towards it creating the tides.

The earth will be consumed when the sun becomes a red giant WAAAY before the moon escapes earth's gravity though.

[mrfish]

someone correct me if i am wrong but i dont believe the earth is going to crash into the sun due to gravity - there are at least 3 bodies affecting the calculation of that and i think that might be one too many to get an accurate prediction -

in 6 bil yrs though the sun will be a red giant and its fuel will be expended in about 10 billion yrs - so really the sun will come to us

run!!!!!!!!!

[Jimdandy]

   

Originally posted by fscott:

Again, only need one question answered ---> Will an object reach a maximum speed in a vacuum based upon it mass?

fscott

The max velocity is independent of the mass. You all remember that in a vacuum a feather will fall just as fast as a 1,000lb ball of led. Acceleration due to gravity it just that. It has nothing to do with mass. EVERYTHING in a vacuum accelerates at 32.2ft/sec/sec on earth. It will NEVER reach a maximum speed with out the presence of some drag/force. A body in motion will remain in motion unless acted upon by and out side force. Terminal velocity is what I think your looking for and that is based on drag (the outside force). Everything has a terminal velocity based on it's Coefficient of drag (Cd). For a man it is roughly 160 mph. F=ma a is constant equaling 32.2ft/sec/sec. So to answer your question no it has nothing to do with mass it has to do with the gravitational constant in a vacuum. Put the object in a medium of some kind and you will have to take into account the shape/drag of the object.

PS Ooops Dinger already answerd you.   You got it Dinger.  

[This message has been edited by Jimdandy (edited 01-29-2001).]

[Tac]

Cc
 

The moon will eventually escape earth's gravity. As it moves away, the earth's gravity pulls it back in which in turn makes the moon's gravity pull the ocean towards it creating the tides.

The earth will be consumed when the sun becomes a red giant WAAAY before the moon escapes earth's gravity though.

An object in vacumm will attain .9999999 (add infinite number of 9's) of light speed. However, the amount of energy you must spend to get it to that speed is not only astronomical, but it also increases exponentially, as object gain mass the as they approach lightspeed. You must have infinite mass and infinite energy to push something TO lightspeed.

Nasa is researching into FTL (Faster than Light) technology..some of the ideas are really wild.

[xmab]

Gee, this is fun. I want to answer the moon question..."Why doesn't the moon accelerate to infinity?"

As everyone has pointed out, there are essentially 2 forces acting on the moon. The first is the attractive force between the Earth and Moon or simply weight. The second is the centrifugal force acting on the moon due to the elliptical orbit (it has to keep changing direction) which points away from the center of the orbit. Now lets sum the forces and see what we get, remembering that the centrifugal force must equal the attractive force for a stable orbit.

First, F=m*a. Now for a circular motion, a=v^2/r where r = radius of the cirle. Thus, Fcentrifugal = m*V^2/r. The attractive force or law of gravitation is Fgravitation = G*m1*m2/r^2 where m1 = moons mass and m2 = earth's mass. So, we let Fcentrifugal = Fgravitation which gives

Mmoon*V^2/r = G*Mmoon*Mearth/r^2

Simplifying gives

v = sqrt(G*Mearth/r)

Lets try some numbers,

G = 6.67*10^-11 m^3/Kg/s^2
Mearth = 5.972 *10^24 Kg
rmoon = 384,400,000 m

which equates to a moon orbital velocity of 1.02 Km/s (which is correct).

So, to sum up. The moon is constantly accelerating at v^2/r or .0027m/s/s. But there is a centrifugal force that opposes gravity that keeps it from going 9.8 m/s/s.

xmab
 

[prz]

answer to the question why an object accelerating cannot reach any speed is more complex and needs solid physics masters course (which I had, hated, A-ed nevertheless and forgot immediately execpt this small burp ;-) I hope this e-mail will not be too heady.

Newton's physics is a "gross" simplification of reality (like any model BTW ;-) but it happens to hold very well @ speeds way below speed of light (up to 0.99c or so). The real formula for speed is not simply time*acceleration but it has further terms which were hellishly complex and funny enough, I sweat the details so go read it up. Basically if you take all the terms, Newton holds up to about 0.99c like I said and then all the other terms in the equations start to kick in and toejam gets weird. First: You get heavier and heavier the faster you get ;-) Literally since e=mc^2 as you know, because you start to build up such a huge amount of energy, your mass goes up so you have to pump even more E to accelerate. If you run the lim (not the fruit, it's a mathematical concept) on the whole thing, you'll figure out that you'll be never able to break speed of light. That formula and the limites were the nobel prize so don't expect to pick it up in an afternoon. Second: time (or rather lack thereof) start to behave ape-shit. Basically you loose the absolute meaning of time but it becomes all relative depending from where you measure stuff, you see the guys you left behind as progressing in their time much slower than they progress in their time. They see you progressing much faster. That's where all the weirdo sci-fi literature with kids coming 8 years back from alpha and finding their great^24-parents comes from. That's why it's all relative ;-)

The question about moon hitting earth I think was answered pretty well by hitech and the other bunch. I though also that moon is actually being decelerated by friction and solar wind. The tide thing confused me, I never heard that it slows the moon down but most of those things boil down to the problem of "multiple masses" and that leads quickly to systems of stiff differential equations and our computing power going "poof" due to the rounding errors. Then basically nobody really has an answer what is cause and what is effect (and as Heidegger said for those who care: "cause and effect and time are just convienent non-existent crutches that our counciousness pulls over reality like a glove to make it accessible to our minds". Pretty heavy stuff, he ;-)

Lastly: HiTech, I really mind you getting me and 2 of my 308 buddies killed in a melee over city yesterday night. If you have to fly a spit well, don't make it so blatantly obvious. Give dweebs a chance next time ... ;-)

[hitech]

sorry xmab, I beg to differ, there is no force opposing the gravity. The vel of the moon tangentail to it's orbit is just changing the direction that gravity is working on the moon, i.e. the moon is constantly falling and accelerating at the rates you specified it just keeps missing the earth. If the moon was not moving, and could pass threw the earth it would fall and come out the other side at the same hieght it started from. This is in ensence what's going on in an orbit except with it's tang. vel to the earth it's ruberband motion turns into one contiuous arc.

HiTech

[popeye]

Picture a bullet fired horizontally.  It is affected by gravity, and eventually strikes the earth.  Now picture another bullet fired horizontally.  This time it is fired from an Hispano 20mm, and is unaffected by gravity, reaches escape velocity and eventually leaves the solar system.


 

[xmab]

HiTech,

Now we have a really interesting question. "Is centrifugal force real?"

As you know, it is really a phantom force. That is, there is no physical construct for it. However, mathematically it exists.

I agree completely with your explanation. It is physically correct. However, how do you explain the limiting case for centripetal acceleration in the moon's case? Is it completely due to the initial condition of the moon's velocity (that is, why hasn't the velocity changed during the existence of the solar system)? Or is the problem in equilibrium due to the balance of weight and centrifugal forces as I have argued?

I submit that my equations predict the limiting tangential velocity for the moon and can predict the orbital velocity for any given radius. Thus mathematically, the notion of centrifugal force is correct, even if there is no physical force that you can feel.

In a way, we are arguing about semantics. But the mathematics are correct and do predict physical experiment.

xmab

[Fury]

My head hurts.

Fury