Expert: Tom Whiting - 11/18/2007

Question
QUESTION: Hello,

  I have always been so much interested to know so much about the universe, infact all about the universe, but never got time or the opportunity to research. I have finally decided to spend atleast some time researching no matter what. I have been having some basic questions which I tried to google but never came across any explanation which really made it clear enough to me. So here are my questions:

1) Why do moon revolve around the earth? I mean why does it not fall on the earth? I understand due the the gravity?

2) Similar question: why does earth revolve around the Sun? why does it not fall?

3) Is there some place above the earth where somehow my body can be placed by which I will also revolve around earth? Or maybe moon? Will I feel the movements?

Regards
Bhavesh

ANSWER: Hi Bhavesh,
Actually the moon does fall "around" the Earth, and the Earth does "fall" around the sun.....but for every kilometer they
"fall" downward..... they also travel forward, due to Centrifical Force,  say, 1000 kilometers. Thus they are actually falling around their respective bodies, and thus maintaining an orbit.
That's basically what an orbit IS around another body....the
centrifical force is exactly balancing out the gravitational
pull downward.  If you could magically stop the moon in its orbit, then it would indeed fall INTO the Earth...or conversely, if you could magically suspend gravity, then the moon would fly off, fly out of orbit on a tangent line to the orbit.  So both
are required, in balance, to maintain an orbit...which obviously it does.  (And has done,  for over 4 billion years).  And same is true of the Earth around the sun, or any other planet around the sun.  Or any moon around it's parent planet.  

Part 3---Yes, anything going into orbit around the Earth,
like those thousands of artificial Earth satellites, are "falling"
around the Earth the same way the moon does....but you
have to get your velocity up to 5 miles per second
(around 18,000 miles per hour)...to achieve orbital velocity,
to keep moving forward fast enough as you fall back toward
the earth, and thus stay in a stable orbit around same.
And the same is true if you wanted to orbit the moon too....
great speed is required to maintain an orbit.  Will you "feel"
any movement?  Only the acceleration up to that high
velocity, called g forces, but once up to speed, no there
is no feeling afterward because you are weightless in orbit, because the centrifical force is exactly balancing the gravitational pull....so there is no feelings of motion.  The only indication that you are in an orbit, would be seeing the slow turning of the Earth below you....but it would be all visual with no internal feelings at all, because you are weightless  (not massless)....just zero weight on a weighing scale, because as you recall, Weight = mass times g....or W = mg.  And once in orbit, g = 0.  And zero times any mass...
gives you a zero weight result.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie PA


---------- FOLLOW-UP ----------

QUESTION: Thanks a lot for your answer. It did give me some basic understanding but now I am even more confused and had to ask you the following to make it even more clear.

You meantioned moon is moving in the orbit which makes it not fall on the earth? My question is why is the moon moving in the orbit? You also mentioned centrifical force, can you explain more on this...I thought centrifical force comes from the rotation not the revolution on the orbit?

You mentioned anything moving in the orbit of the earth are falling around the earth in the same way like the satellites. So are these satellites actually moving that fast to avoid falling? In which deriction they are moving?

Answer
Hi again Bhavesh,
In reverse order, Yes, the bodies in orbit around another
body are moving so fast that they do not "fall into" the other
body that they are orbiting.....that's the basic definition of
an orbit. It's a balance between gravity and centripetal
acceleration of circular motion.  And actually, when you study it further, both bodies are actually orbiting around a common center of gravity axis.....there is not really one body stationary, and the other one doing all the moving.  {There is no such thing as absolute REST in the Universe ----  everything is moving,  relative to everything else,  in our Universe!)!!
   For instance, the Earth-Moon system is actually both bodies revolving around a common center of gravity axis, but since the Earth is so much more massive than the moon, that axis of revolution is actually located well below the Earth's surface.  So we don't notice that we are also orbiting the moon around a common center of gravity.

Which way are artificial satellites moving?  They are either
launched in a North--South direction (called a polar orbit)
Or they are launched eastward, taking advantage of the 1000 mile per hour Earth's rotation,  so they are already traveling
1000 miles per hour eastward even before launch, just because of the daily Earth's rotation, measured at the equator.
A westbound launch would be a waste of fuel just to first
overcome the Earth's rotation, and then accelerate up to the
required 5 miles per second orbital speed.  And an East-West
orbit is called an equatorial orbit, and no one cares whether
it's from west to east, or east to west...so we always launch
eastward to take advantage of the Earth's daily rotational
speed.

Part I....NO!!, centrifical force, or more accurately called
Centripetal Acceleration  {a  =  v squared/r   or,   velocity squared divided by the  radius of the circle)
applies whenever their is ANY circular motion involved, be it rotation, revolution, or just you swinging around a ball attached to a string over your head.   Your problem in understanding all this,  isn't astronomy , it's in BASIC  physics.  You need to find, and read, a basic physics textbook ...usually "acceleration in circular motion"  is covered in Chapters 3 and 4...very early on, as it is a very basic physical property, just like Newton's 3 laws of motion.
I cannot cover all the laws of physics with you in these short 5 minutes...you have to read and understand it yourself with a basic physics textbook.

Suffice to say that anytime there is ANY circular motion, a force is produced that is opposite that of gravity pulling inward, such that the two balance each other, and therefore a stable orbit is maintained. And yes, it takes a high velocity to do this.   So no, centripetal acceleration comes into play with revolution of bodies too, anytime there is circular
motion, be it rotation on an axis, or revolution about an axis.

Why is the moon moving?  Because everything moves....as
stated earlier, there is no such thing as absolute rest in
our Universe....everything moves relative to everything else.

Oh, for future reference, science does not answer the questions "WHY  and  WHO?"  That's for theologians to answer...WHY are we humans here,  WHO put us on this Earth?  These are religious or philosophical questions......not
science questions.....
Why is Saturn a planet in our Solar System?
Why does the moon orbit the Earth?

We in science are only prepared to answer "HOW? and By What Process"....did that occur?
That's the basic difference between science and religion....
And we stay out of their area of interest (the why and who?)
questions, and hopefully, they stay out of our territory,
the HOW and by what process, questions.  
They are the experts on the... who and why....and we in
science are the experts on the HOW and by what process,
questions.
Hope All this helps,
Clear Skies,
Tom Whiting