- #71
Drakkith
Mentor
- 22,932
- 7,301
maverick_starstrider said:Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.
Lol, awesome.
maverick_starstrider said:Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.
maverick_starstrider said:Basically the entire sentiment is awesomely summed up here: http://www.xkcd.com/895/ that comic had me laughing so hard.
Polyrhythmic said:True, xkcd is awesome!
Drakkith said:If my rudimentary understanding of GR is correct, it is because the ball and the torus are moving through spacetime. The curvature of spacetime by mass causes the ball and torus to move closer to each other. There was nothing needed to get it all started because they were already moving.
danR said:I don't like to talk about 'forces', because my university course said they were replaced by field-theory even back in the 19th century, but why do I feel there is still some kind of 'force' needed to kick-start things?
Polyrhythmic said:I'd rather say that field theories describe forces, rather than replace them.
danR said:Could we say they 'explain' forces? That forces are the surface manifestations of the underlying machinery of fields?
Could gravity be a force after all, in that sense?
Polyrhythmic said:I'd say yes and yes. Where the second question leads us back to what has already been discussed in this thread.
danR said:Only in the special case of an infinitesimal drop, and selecting the same 'vertical' line from centre to perimeter. As either falls, it will veer from centre and bang obliquely into your instrument panel, since the perimeter is moving much faster than the initial sideways motion of the object.
If they are 'dropped' side-by-side, they will 'fall' at the same rate, but divergently. On Earth they will fall convergently toward the centre of the earth. Hence, only choosing the same vertical line for each. You have to run the experiment separately for each.
So gravity and centrifugal 'force' are rather crappily similar. As long as everything stays put, and is all piled up at the same place, are they the same.
Mitch Rowe said:OK.. That may be true. But what if the spaceship were as big as a galaxy or an orbit around the Earth? And we only had 10 feet of vertical space to drop the objects. Now we can drop them anywhere and there is no difference.
And I think I have just proved to myself that "Gravity" and "Centrifical Force" are essentially the same.
This is how:
When an Astronaut is in orbit he is essentially weightless. Why? Because he is falling towards Earth but has enough velocity to keep himself from spiraling inwards. If he begins to lose orbit he can increase velocity to maintain it.
So what would happen if another spaceship came by and loaded cargo on to his ship so the ship now has twice as much Mass? Even though to him the extra Mass is essentially weightless shouldn't he have to increase his orbital speed to keep from falling? Actually I guess the spaceship will increase its own velocity due to the fact it has twice as mass for Earth to pull. No.. Thats not right I don't think?. If everything falls at the same rate I guess nothing happens if the spaceship suddenly has twice as much mass other than it begins to fall I suppose. So I suppose the Astronaut must increase speed to counteract Earths gravitational pull?
Would that be right?
danR said:For your second part, he has only added mass that already had an identical orbital configuration as his own. This tells me nothing about centrifugal/gravity equivalence.
No.Mitch Rowe said:OK but wouldn't the speed of the orbit need to increase to maintain the orbit??
Mitch Rowe said:OK.. You don't have to participate. But I invite anyone to argue with me as I have an idea that I will be getting to if I am not shot down before then.
danR said:I came back for a minute to see about something I asked (PF is infuriatingly addictive). The trouble is that you are so nearly right (except the astronaut added-mass thing), that you may think you are entirely right.
But that's not good enough for experts. If something is nearly right, they want to see the part that is totally wrong also. I understand there are theories of gravity/inertia/acceleration that demonstrate equivalence under highly restricted circumstances. This entails a mastery of things like manifolds, tensors, gauge-theories, Higgs particles or something likewise esoteric. To understand those theories takes both of us way beyond our abilities to even start to understand.
DaveC426913 said:No.
A wrench massing 500g will happily orbit alongside a space shuttle massing 80 tons. As we know everytime we watch a video of things floating inside (or outside) the shuttle.
If you had a 500g wrench and someone came along and added 79,999 more wrenches to it, it would just keep orbiting at the same speed and altitude*.
*Note btw, that the someone came along carrying those 79,999 extra wrenches, had to bring all that momentum with them. They accelerated each and every wrench up to orbital speed before rendezvousing with wrench zero.