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weight of a 44,000-mile-tall man

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ongexi
1034493.  Sun Nov 10, 2013 8:51 pm Reply with quote

I thoroughly enjoyed the episode 'Kinetic'. However, it is not correct that a 44,000-mile-tall man standing on the Earth's surface has no weight. Indeed, if he had no weight (which is the centripetal force in this example) then he couldn't orbit the Earth at all, but would fly off into space in a straight line.

The concept of "weightlessness" in, for example, a spaceship orbitting the Earth or a plane in freefall is often misunderstood. The person occupying the vehicle certainly has weight, but they are falling (orbitting is just falling but with a certain high horizontal velocity) at the same rate as the bathroom scales on which they might try to stand, thus giving the illusion of weightlessness.

To put it more concisely, weight is the gravitational force acting on the person.

 
CharliesDragon
1034537.  Mon Nov 11, 2013 8:10 am Reply with quote

Well, they might not have explained it perfectly, but I understood that they meant he would appear to have no weight. Generally speaking, things that have mass also have weight, although you might not be able to measure it in the current state or you'll get different results depending on gravity.

The Earth is pretty damn heavy, but it doesn't fall down into a cosmic valley or something, because there's nothing to pull it into it. (Unless the entire solar system is being pulled into it, but as far as I've gathered, we're being flung away from other solar systems and galaxies.)

Now I'm gonna sit here and wait for people to nitpick my wording and drag in a lot of physics, maths and other sciences I don't understand. Knock yourself out, guys!

 
ongexi
1035374.  Thu Nov 14, 2013 2:35 pm Reply with quote

Hey CharliesDragon,

Some good thoughts there. I'd probably counter by saying that QI is a program that makes a big deal about highlighting people's misconceptions, and the exact wording of their questions to the panel is often deliberately critical. So when they get it wrong on either or both of these counts, I am sure they can accept it being pointed out. All in a good-natured way of course.

Regarding your other points, the more obvious effects of the Earth's weight are that the Earth is in orbit around the Sun (if the Earth had no weight it would fly off out of the Solar System; you can think of the Sun here as being your cosmic valley (or better, cosmic hole)), and that the Earth has tides (different parts of the Earth have different weights depending on how far the parts are from the Moon). Here I am trying to think of examples that focus on the Earth being pulled by something rather than the Earth pulling on something, although gravity is in reality about two objects mutually pulling each other.

Although it is true that galaxies are typically moving away from each other (lots of observational evidence), I haven't heard that stars and solar systems within a single galaxy (like our own Milky Way) are typically moving away from each other. I am pretty sure I would have heard about it if that was the case. For instance, it would result in an individual galaxy flying apart, and I am fairly certain there is no evidence of that except in the case of galaxies colliding.

 
PDR
1035406.  Thu Nov 14, 2013 5:56 pm Reply with quote

All stars in the universe are moving away from all other stars in the universe all the time - that's what we mean when we establish that the universe is expanding.

There is another thread discussing the "weight of the 44,000 mile man" thing here (this part of the discussion starts at the bottom of the first page).

PDR

 
ongexi
1035528.  Fri Nov 15, 2013 8:07 am Reply with quote

PDR wrote:
All stars in the universe are moving away from all other stars in the universe all the time - that's what we mean when we establish that the universe is expanding.

There is another thread discussing the "weight of the 44,000 mile man" thing here (this part of the discussion starts at the bottom of the first page).

PDR


Cheers PDR. I like it here though. :)

Can you link to research for your first statement for me? I am interested in it. Every time that I have heard of evidence of expansion of the universe it has been in terms of the red shifts of galaxies. Although, true, every time that I have heard the expansion explained it is in terms of new space being created between old space, if you get my drift. The latter would suggest you are right in saying that, on average, stars of an individual galaxy are moving away from each other, but I am going to guess that we don't have observational evidence of that, at least yet. Naturally it is much easier to measure redshifts of galaxies at large distances than to determine the small mean redshift of closer stars, and that may expalin the lack of observations. But let's say that you are right, and that the individual stars are, on average, moving away from each other (and likewise the Earth is moving away from the Sun because of universal expansion). In that case, how are solar systems and galaxies remaining stable? With expansion, the bodies are further apart, the effect of gravity is weaker, and hence the systems (solar systems or galaxies) should fall apart.

 
ongexi
1035530.  Fri Nov 15, 2013 8:10 am Reply with quote

Also, I am guessing that you don't literally mean "All stars in the universe are moving away from all other stars in the universe all the time," since there are plenty of stars moving towards our solar system, and indeed galaxies moving toward our own galaxy.

 
chrisboote
1035853.  Sun Nov 17, 2013 5:47 am Reply with quote

ongexi wrote:
Also, I am guessing that you don't literally mean "All stars in the universe are moving away from all other stars in the universe all the time," since there are plenty of stars moving towards our solar system, and indeed galaxies moving toward our own galaxy.

Really? I thought that everything was moving away from us - and indeed everything else - which, unless we really ARE the absolute centre of the universe, is one of the proofs of the expanding universe theory?

 
ali
1035864.  Sun Nov 17, 2013 7:19 am Reply with quote

chrisboote wrote:
ongexi wrote:
Also, I am guessing that you don't literally mean "All stars in the universe are moving away from all other stars in the universe all the time," since there are plenty of stars moving towards our solar system, and indeed galaxies moving toward our own galaxy.

Really? I thought that everything was moving away from us - and indeed everything else - which, unless we really ARE the absolute centre of the universe, is one of the proofs of the expanding universe theory?


In general, this is true. In particular cases though, it is not. For example, the Alpha Centauri system will be approaching Sol for about the next 28,000 years or so (due to the particular properties of the two systems' galactic orbits). On a galactic scale, light from M31 (the Andromeda Spiral galaxy) is blueshifted rather than redshifted, indicating that it is moving towards us, rather than away. Again, this is due to local (quite a large locality :D) gravitational effects.

 
gruff5
1035866.  Sun Nov 17, 2013 7:42 am Reply with quote

As far as I understand it, the expansion of space is only evident on the largest scales, where galaxies or clusters of galaxies are, on average, "hanging in space", rather than being under a net gravitational pull in one direction or another.

The interstellar space within a galaxy is theoretically, though not demonstrably, also expanding, but the small effects of that expansion are overwhelmed by the strong local gravitational attractions that keep the stars in their orbits around the galactic centre. This is even more the case for a solar system.

Dark energy is supposedly accelerating this expansion. At some time in the distant future, the expansion may become so rapid that it overwhelms local gravitational attractions and rips the cooling embers of galaxies and solar systems apart. Ultimately, the expansion of space may rip atoms apart. Not sure I believe all of this!

 

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