# Homework Help: Simple Concepts, Having Trouble

1. Feb 26, 2004

### holly

I have a test on Monday. I am still having trouble with the following concepts. ***Please remember I am just taking the physical science kind of physics.***
Q. For the astronauts inside the orbiting space shuttle, there is no force of Earth's gravity acting on them. True or false?
A. Answer given by book: True. Answer given by professor: False. I can follow him, not the book. But I want to be able to recognize this question's main point in case it's on the test in a different guise. There are two legitimate attitudes in physics about weightlessness?

I can't sort out why the book says one thing, the professor another.

Another kind of question involves rocks thrown upward. In general, when a rock is thrown upwards at any angle, the velocity decreases so far as the vertical component is concerned, correct? The horizontal velocity doesn't decrease, correct?

Thank you for any help.

If I have seen farther than most men, it's because I stood on Leibnitz's neck. -- I. Newton

Last edited: Feb 26, 2004
2. Feb 26, 2004

### FZ+

I get the impression that the book is wrong. Obviously the gravity acts - else, they simply would not be in an orbit.

In the simplified case of no air resistance, yes. (Though decreases or increases of velocity depends on the direction you are considering the movement as positive.)

3. Feb 26, 2004

### Tom Mattson

Staff Emeritus
The professor is right, and the book is wrong. The force of gravity on the astronauts in orbit is not much less than it would be on Earth. As FZ said, gravity is what keeps the ship in orbit. If it weren't acting there, then the ship would just travel in a straight line.

Yes, but not in quite the way you think. It is not the case that the answer to the above question is "true and false". What is going on is that the astronauts on the ship are in free-fall. That is, they are falling at precisely the same rate as the ship, which gives the appearance that they are weightless. You could achieve the same effect right here on Earth by cutting the cord in an elevator (don't try it).

It is because the author of the book is a dimwit.

Correct. The vertical component decreases, and the horizontal component remains constant (zero, to be exact).

4. Feb 26, 2004

### holly

Thank you for answering. We don't consider air resistance in our class.

Read your blog. Sorry it's cold where you are. It's sunny and warm where I am, "the best-kept secret in America," the cleanest-living, most inexpensive, sunniest, best-mannered and practically the prettiest city you could ever see.

Thx again. I think the thing about things being weightless IS considered right in some circles. I think the shuttle has weight but the guys and gals inside are weightless, to some. I have to look back on another thread to find out what that theory is called.

5. Feb 26, 2004

### Tom Mattson

Staff Emeritus
That depends on how you define "weight".

If you define it as "the force exerted on a body by the Earth", then the astronauts are not weightless.

If you define it as "the reading on a scale that is stationary relative to the body being weighed", then the astronauts are weightless, because the scale will read zero.

However, you were not asked if the astronauts are "weightless". You were asked if there is a nonzero gravitational force acting on them from the Earth, and there is!

The book is dead wrong.

6. Feb 26, 2004

### turin

Hmm. Your location says "west texas." Based on the description, I don't believe that you are in the Czech town of West, TX (I-35 just north of Waco), or that certainly would be a well kept secret. Perhaps, if you include the east side of the hill country as part of west texas, then I think I know the town where you are (although I wouldn't call it a secret for any Texan).

7. Feb 26, 2004

### holly

Ha ha, you're way off in identifying my city! I wouldn't walk three feet to spit on the Hill Country, btw. Lived there already (bluebonnet, schmoo-bonnet), moved here, lived all over the U.S., all three coasts, the middle, the appendages, on islands, mountains, bayous, and this place is the BEST. Other Texans, being confused, laugh at our faire towne.

HOWEVER, it does lack in physics courses, ha ha. I am having trouble with this business of the moon versus the sun in causing tides, this business of it being the DIFFERENCE "in pull" that causes it...lots of hypothetical questions about it that I'm getting wrong. Surely, if the sun is pulling on us so much greater than the moon, IT is the one causing more of the tides? Since it is greater, that would be the greater PULL, would it not?

8. Feb 26, 2004

### turin

Well, I'm stumped. It will remain a secret to me. Personally, though, I don't think I would call any town that wasn't on the ocean a beautiful one.

Tides are a second order effect. It isn't which one is pulling more strongly, but which one's field varies more greatly from the near side of the Earth to the far side. Since the Earth is "floating around" out in space, then it is in freefall with the sun and the moon. In other words, the force, as measured WRT the earth, is null. But this is the same frame that the oceans use, so they don't see any first order effects (the oceans don't directly feel pulled towards the heavenly bodies because they are already falling towards them with the Earth). A falling elevator on the sun "feels" the same as a falling elevator on the moon (to first order, and neglecting temp).

Since Fgrav ~ r-2, then the far side isn't being pulled as much as the near side. And the average pull (the one responsible for the actual orbit) is somewhere in between. So, the center of the Earth falls towards the sun at a certain rate. Whereas the near side falls "faster" and so gets bulged out. And the far side falls "slower" and so gets "left behind" and bulges out.

Since the moon is closer, the drop off in r-2 is relatively more pronounced from one side of the Earth to the other. I'm not sure, but I think that the tidal field due to the moon is a lot stronger that the one due to the sun. The tidal field due to the other planets and stars is quite negligible.

Last edited: Feb 26, 2004
9. Feb 27, 2004

### marcus

LOL

how are things going?

is PF working out for you in the sense that
the discussion contributes to your understanding

are you all right about tides now or do you want
more talk about that

10. Feb 28, 2004

### marcus

see that when something really matters you use traditional
units---not metric

wouldnt walk three feet to spit works better as speech, to the ear, than
wouldnt walk one meter

----------you say-------------
I am having trouble with this business of the moon versus the sun in causing tides, this business of it being the DIFFERENCE "in pull" that causes it...lots of hypothetical questions about it that I'm getting wrong. Surely, if the sun is pulling on us so much greater than the moon, IT is the one causing more of the tides? Since it is greater, that would be the greater PULL, would it not?
-----------------------------

the size of the "pull" doesnt determine the tidal effect

(or else the sun would cause higher tides than the moon, because at where we are the pull of the sun is roughly some 200 times stronger than the pull of the moon, so it would be raising 200 times bigger tides!)

a low hill can be steeper-sided than a high mountain
its not always like that but it can happen
near where I live there is an example of it
there is this rather low hill that is
nevertheless so steep
that people have built stairs up and down it

and about 25 miles from us there is an old mountain that has very gradual slopes-----overall it is higher than the little hill, but more spread out so that it is gentle-sloped and rises and descends more gradually

-----------------
the pull of the sun, at this distance from it, is quite strong
but it slopes off with distance only very gradually
it wouldnt be appreciably less if you went another 8,000 miles further out from the sun than we are

i happened to mention 8000 miles because that is the diameter of the earth and it helps me picture to think of going one more earthdiameter out away

the pull of the moon, at the distance we are from it, is much weaker
but it drops off more noticeably with distance.

we are 240,000 miles from the moon
and if you backed off till you were 8,000 miles farther
(a total of 248,000) then the pull (which is already weak)
would be considerably less

i guess that is because 8000 miles is more of a significant percentagewise difference vis a vis the moon

the tides are caused not by the brute STRENGTH of the pull but by the difference in pull between one side of the earth and the other.

the waters of the earth (imagined as one big eggshape ocean) bulge towards the moon on the side towards the moon because of the stronger pull
and bulge away from the moon on the side away from the moon because of the weaker pull
and so the one big ocean is roughly eggshape
dont they draw you a picture in your textbook

sometimes I think it is you who are the PhD physicist and not your fabled ex-husband
this remark about standing on Leibniz' neck is sophisticated and very funny
In my experience usually only people who understand physics know what a self-righteous sadistic jerk newton was and how mean etc. he would have gladly stood on Leibniz neck and not stopped at that either
so I am seriously wondering if you needed that little sermon on tides

Last edited: Feb 28, 2004
11. Feb 28, 2004

### holly

If a person's circle was mainly physicists and mathematicians, they just pick up these names and such like Leibnitz et al. I rescued kitties quite often, and for years I was surrounded by a horde of them with such names as Alpha, Beta, Gamma, Cantor, Keppler, Al, Pico, Nano (undersized kitty), Zeno (wouldn't come when called), Dopple-Paws, Gaussie, etc etc. So, I'm like a parrot, squawk squawk, sensible remarks I overheard, squawk squawk.

I am still having trouble with the tide business, despite the excellent teachings and the drawings in the book of the earth with a bulgy side and a smiling moon and sun causing spring tides. There is a question about if the moon were gone, how many tides a day would we have? Is it a trick question? If the moon were gone, wouldn't we sneak closer to the sun and thus evaporate the seas? And there is another question about if the moon were twice as massive but the same size around, would the tides be the same, be absent, be doubled, or be quadrupled? Wouldn't it be pulled into the earth? And then there's a question about if everything in our galaxy were twice as massive yet the same size, would anything change? And there's a question about explaining why lakes don't have tides. They do too have tides! The Great Lakes do! I don't like mixing the real world with the imaginary one. Just when I grasp one point, another point is introduced and I am back to square one.

12. Feb 28, 2004

### marcus

first, if your course is ending and you will not be returning here, we will miss you

second, I think Leibniz is spelled without the T

third, the cats made me understand something about where the science culture was coming from and BTW you have outdone even your usual high standard in writing this batch of questions

If I have seen farther than most men, it's because I stood on Leibniz's neck.

If I have seen less than others, it's because some giant's shoulders are always in the way.

vaya con dios wacky one. you improved things for a while

Last edited: Feb 28, 2004
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