|
Post by fiveonit on Mar 13, 2010 19:00:55 GMT -4
If the Earth ever decides to turn itself into a neutron star or black hole, then we'll all have to start worrying about tidal effects. I don't see that happening anytime soon, so I'm not losing any sleep over it.
|
|
|
Post by randombloke on Mar 13, 2010 19:59:16 GMT -4
Wouldn't the tidal forces be stronger because you're accellerating away from Earth? Maybe? But why would that be a problem? Since the tidal difference between your feet and your head is so small as to be un-measurable, multiplying it by a couple orders of magnitude still won't amount to much. You really need to realise how pitifully weak gravity when is compared to the other forces. After all, you can generate electromagnetic effects capable of defying Earth's gravity more or less indefinitely with but a thought. Or less even; every time you lift up an apple.
|
|
|
Post by BertL on Mar 15, 2010 1:48:53 GMT -4
I'm aware of that, randombloke. I was just wondering.
|
|
|
Post by ka9q on Mar 15, 2010 5:12:05 GMT -4
Wouldn't the tidal forces be stronger because you're accellerating away from Earth? The only time you accelerate during an Apollo trip to the moon is during an engine burn, e.g., translunar injection (the second burn of the Saturn S-IVB) or lunar orbit insertion (the first major burn of the service propulsion system in the service module). These burns (all rocket burns, in fact) create an acceleration within the cabin that are perceived just like gravity, though it may be greater or less than the 1 g you experience on the ground depending on the thrust of the rocket and your current mass. At all other times in the flight you're in free fall and (mostly) weightless even though you may be picking up or losing speed with respect to the earth, moon, or some other astronomical body. Why "mostly"? One of Einstein's famous thought experiments about relativity is to imagine yourself inside a closed elevator car. If you experience what seems like earth gravity you can't conduct experiments or otherwise tell, without looking outside, whether you're sitting motionless on the surface of the earth or are in deep space above a rocket engine accelerating you at a steady 9.8 m/sec^2. And if you experience weightlessness, again you can't tell without looking outside if you're floating in deep space or free falling onto the earth. You might think you'd discover the latter case soon enough, but not if you're actually orbiting the planet; an orbit consists of free-falling all the way around a planet without hitting it. Einstein concluded that you can't tell the difference between gravitational acceleration and acceleration by a force because gravitational mass and inertial mass are one and the same. But there's a caveat. You can detect the presence of a nearby planet without looking outside your elevator car: by looking for its gravity gradient, a position-dependent difference in acceleration. If your elevator car is in deep space being uniformly accelerated by a rocket, every point within your car experiences the exact same acceleration. But if you're standing vertically on the earth's surface, your feet will experience a very slightly greater gravitational acceleration than your head. I.e., each gram of mass in your feet will experience a slightly greater downward force (weight) than each gram of mass in your head. This shows up as a very small net force tending to stretch your feet away from your head. This remains so even if you're in free fall (including orbit). Most - but not all - of what you perceive as "gravity" will be gone. Only your center of gravity - a point - experiences true zero gravity. Every other point experiences a very slight acceleration as your rigid elevator car forces them to move in slightly different paths in space than they'd follow if they were free to move independently. These small forces are exactly the tidal forces we've been talking about. They're strongest near a massive body and decrease as you move away from it. It's why NASA refers to the environment inside the ISS and shuttle (which are unable to move above low earth orbit) as "microgravity" rather than "zero gravity". And tidal forces are unaffected by the thrust of a rocket because they're differential forces, while a rocket accelerates every point within its spaceship equally. Now it's true that if you strap a big rocket engine to a large and fragile object in space, the resulting acceleration might damage it when the smaller gravity gradient (tidal) forces were insufficient to do so, but they are still two different kinds of forces. If you were to do gravity gradient experiments on an Apollo flight to the moon, you'd notice that the microgravity forces seen in low earth orbit decrease as you gain altitude, only to increase somewhat again as you approach the moon. If you could escape the solar system entirely, you might finally be unable to detect any microgravity forces and you'd finally experience true "zero" gravity. Gravity gradient forces may seem trivial, but they're actually very significant in spacecraft design. Some spacecraft use gravity gradient forces to stabilize themselves without spending fuel or energy, typically by deploying a mass on a boom either toward or away from the earth. The Long Duration Exposure Facility (LDEF) was a bus-sized spacecraft with mostly passive experiments that used gravity gradient stabilization. Even spacecraft that use active attitude control have to account for gravity gradient forces to avoid unwanted torques that may require propellant and/or energy to compensate.
|
|
|
Post by JayUtah on Mar 15, 2010 11:03:23 GMT -4
Only your center of gravity - a point - experiences true zero gravity. Hence the nit-picky difference between center of mass and center of gravity. For practical purposes they are the same geometric point. But their physical definition differs, and they diverge only when gravity tides are not a low-order force.
|
|
|
Post by ka9q on Mar 15, 2010 13:02:28 GMT -4
Quite true. But even Apollo documentation regularly speaks of the "center of gravity" (or "c.g.") rather than the more correct (and unambiguously defined) center of mass; see the "Mass Properties" section of any mission report.
Interestingly enough, this was one of the sections often classified "confidential" in the earlier mission reports. I wonder why. (All have long since been unclassified.)
|
|
|
Post by JayUtah on Mar 15, 2010 18:28:48 GMT -4
Quite true. But even Apollo documentation regularly speaks of the "center of gravity" (or "c.g.") rather than the more correct (and unambiguously defined) center of mass...I use the terms interchangeably too when I'm not careful. "Center of gravity" is more widely used in lay terms and that bleeds over into professional writing. Interestingly enough, this was one of the sections often classified "confidential" in the earlier mission reports. I wonder why. (All have long since been unclassified.)Because the performance of the launch vehicle can be inferred from the mass properties of the payload, and the performance of the Saturn V was not something we wanted the Soviets to know about in much detail.
|
|
|
Post by Count Zero on Mar 15, 2010 21:00:18 GMT -4
IIRC, the gravitational gradient contributed to Skylab's early orbital decay. When it was no longer actively stabilized, tidal forces brought the long axis in line with the center of the Earth. This caused increased drag in the rarified upper atmosphere (which had thickened due to solar activity). This brought the station down before the long-delayed Shuttle could go and boost it. I always thought it was interesting that Babylon 5 had its long axis perpendicular to the planet it orbited. Although the station appeared to be too high to be affected by atmospheric drag, there would still be utility in minimizing the tidal effects within the station. Of course, it was probably just done for aesthetic reasons, but I've always had the Marvel "No-Prize" mindset of "don't tell me that it's wrong - Think of a reason why it is right."
|
|
Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
|
Post by Al Johnston on Mar 16, 2010 5:22:43 GMT -4
I always thought it was interesting that Babylon 5 had its long axis perpendicular to the planet it orbited. Although the station appeared to be too high to be affected by atmospheric drag, there would still be utility in minimizing the tidal effects within the station. Of course, it was probably just done for aesthetic reasons, but I've always had the Marvel "No-Prize" mindset of "don't tell me that it's wrong - Think of a reason why it is right." Did it? I always got the impression that B5 was fairly randomly oriented wrt Epsilon III, or more commonly, oriented so that Ivanova could get a good view of that week's major threat/plot point. After all "... Ivanova is always right. Ivanova is God." ;D
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Mar 16, 2010 8:05:06 GMT -4
I think Babylon 5 would have to be oriented with its long axis parallel to the planet's surface (i.e. along its direction of travel) because of the vector along which spacecraft approached it's docking bay. B5's attitude was stabilized with thrusters, which was made clear in one episode when something (an explosion, I think) caused the station to lose attitude.
|
|
|
Post by Data Cable on Mar 16, 2010 8:05:15 GMT -4
After all "... Ivanova is always right. Ivanova is God." ;D "Trust Ivanova, trust yourself... anyone else, shoot 'em!"
|
|
|
Post by Jason Thompson on Mar 16, 2010 8:29:57 GMT -4
quote] The only time you accelerate during an Apollo trip to the moon is during an engine burn, e.g., translunar injection (the second burn of the Saturn S-IVB) or lunar orbit insertion (the first major burn of the service propulsion system in the service module). [nitpick mode] Since velocity on a spacecraft is never constant (with gravity a] slowing you down and b] pulling your trajectory into a curve) you are constantly accelerating on a trip to the Moon... [/nitpick mode]
|
|
Jason
Pluto
May all your hits be crits
Posts: 5,579
|
Post by Jason on Mar 16, 2010 11:29:27 GMT -4
quote] The only time you accelerate during an Apollo trip to the moon is during an engine burn, e.g., translunar injection (the second burn of the Saturn S-IVB) or lunar orbit insertion (the first major burn of the service propulsion system in the service module). [nitpick mode] Since velocity on a spacecraft is never constant (with gravity a] slowing you down and b] pulling your trajectory into a curve) you are constantly accelerating on a trip to the Moon... [/nitpick mode] True, but since your acceleration is due to gravity you won't feel the acceleration.
|
|
|
Post by BertL on Mar 16, 2010 12:22:21 GMT -4
Or to be a bit more complete: since the acceleration applies to you and the spacecraft equally and comes from an outside force you do not notice the acceleration.
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Mar 16, 2010 12:59:14 GMT -4
[nitpick mode] Since velocity on a spacecraft is never constant (with gravity a] slowing you down and b] pulling your trajectory into a curve) you are constantly accelerating on a trip to the Moon... [/nitpick mode] The trajectory is also never a straight line. Even a satellite traveling in a circular orbit at constant speed is constantly accelerating. Velocity has both speed and direction - changing either means a change in velocity, and thereby, an acceleration.
|
|