# About space and gravity, too long to explain in thread title **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [December 12, 2024, 3:14pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558 "2024-12-12T15:14:46Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 13, 2024, 5:11pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/21 "2024-12-13T17:11:35Z")
> [@RitterSport](#): > > Minnesota, land of 10 kiloponds! Lol, nice one. I still hear _kips_ every once in a while from an old timer, it’s 1000 pounds or pounds-force. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 13, 2024, 5:42pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/22 "2024-12-13T17:42:14Z")
> [@Schnitte](#): > > This sort of confusion is precisely the reason why there is a non-SI but useful unit of weight, the kilopond, defined as the weight of one kilogram on the surface of the Earth - but with a name that clearly sets it apart from the kilogram as a unit of mass. Useful for whom? I have a little digital scale which internally measures who-knows-what, but the readout is in grams. In fact, I cannot think of a common item or specification that uses that unit. (And some common items use weird units, like strength specified in kilopounds per square inch.) ETA > [@jnglmassiv](#): > > I still hear _kips_ every once in a while from an old timer, it’s 1000 pounds or pounds-force. I don’t know if they are that old. I was thinking of how you grab a welding electrode and there is a specification on it like E7018, where “70” means 70,000 pounds per square inch. ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [December 13, 2024, 7:08pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/23 "2024-12-13T19:08:23Z")
> [@DPRK](#): > > Useful for whom? For thrust-to-weight ratios for rockets and aircraft engines, for instance. For such applications, it’s neat to have a unit of force that can easily be put into relation to a unit of mass. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 14, 2024, 6:27pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/24 "2024-12-14T18:27:56Z")
I suppose one has to massage the units somehow. I read a book where the author decries the practice of specifying specific impulse in seconds. Explicitly using “kiloponds” relies on a magic number in the same way. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [December 14, 2024, 7:01pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/25 "2024-12-14T19:01:45Z")
> [@Schnitte](#): > > If the smaller object is moving tangentially to the larger sphere at 0.333 mm/s (0.013 inch/second) at that distance, it will orbit it; if it’s slower, it will fall towards the larger sphere; if it’s faster but slower than 0.47mm/s (0.019 inch/s), it will rise to a higher orbit; and if it’s faster than 0.47 mm/s it will fly away into infinity. You are assuming circular orbits, but I believe that for most speeds between 0.333 mm/s (0.013 inch/second) and 0.47mm/s (0.019 inch/s) at ten cm separation the orbits will be elliptic and the speed and the separation will not be constant. ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [December 14, 2024, 10:03pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/26 "2024-12-14T22:03:10Z")
Well, for each speed in that range you can calculate a circular orbit that has that exact speed as orbital velocity. But for sure it’d also be possible to have elliptical orbits whose orbital speeds vary within that range. ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [December 18, 2024, 10:37pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/27 "2024-12-18T22:37:08Z")
> [@HeyHomie](#): > > Let’s say I have the same sphere with me on the ISS. If I tried to move it, would I essentially just be pulling myself towards the sphere, since its mass is so much greater than my own? If the floor of the ISS were at my feet, and I was scrunched down between the floor and the sphere, would I be able to lift it? Assuming you can get underneath it and exert a constant force of 890 N (200 lb) with your legs for a full second while braced against something like the ISS floor, F=ma says you’d be able to accelerate a 1000kg floating sphere to .89 m/s (2.9 ft/s) over this distance (~1.5 ft). ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 18, 2024, 11:04pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/28 "2024-12-18T23:04:51Z")
> [@jnglmassiv](#): > > Weighing is how we determine mass (for 1000 kg spheres) and it’s assumed by convention that the measurement was performed here on the surface of Earth. I built a neat device that measures mass directly. Of course there’s no way to _intrinsically_ measure mass, but the relationship F=ma is reliable at low speed. My first revision applied a certain force to an object, and I then measured the acceleration, which together gave me the mass. For the second revision I decided to hold acceleration constant and measure the force instead, which worked just as well. But the third revision was the best, where I decided to make use of an ambient constant acceleration field, meaning I only had to measure force, which dramatically simplified the device. I call it a “scale”. A 1000 kg object will weigh 1000 kg on Earth or the Moon or anywhere else. The scales have to be calibrated to the local acceleration, but that’s no different than calibrating them for any other reason, like how many newtons it takes to deflect a spring by so many millimeters, etc. And that calibration will be wrong in another environment, like one at a different temperature. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 18, 2024, 11:15pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/29 "2024-12-18T23:15:19Z")
It should also be mentioned that, according to some models, gravity does behave differently at sufficiently short distances (where “sufficiently short” might plausibly be as much as a few millimeters). But this is very difficult to test with Cavendish-like experiments, because the closest together you can get two spheres is the sum of their radii, and for the force to be detectable, the spheres have to be fairly big. Of course, “difficult to test” is not the same thing as “impossible”, and so some researchers have come up with very clever ways of arranging the masses, and detecting very small forces, and reducing all of the many sources of noise, and so on, in order to test gravity at shorter and shorter length scales. So far, they haven’t detected any deviation from Newton’s Law, but there’s always the possibility that the next experiment, at shorter length scales and with more precise measurements, will. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 19, 2024, 12:55am UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/30 "2024-12-19T00:55:28Z")
We really, really need stable white dwarf matter for this and a number of other applications. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [December 22, 2024, 10:45pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/31 "2024-12-22T22:45:13Z")
> [@TriPolar](#): > > Don’t want to sidetrack things but I remain confused about why mass and inertia are different since they’re directly proportional to each other. The question is whether gravitational mass (the _m_ in the formula for gravitational force) and inertial mass (the _m_ in the formula for momentum or kinetic energy) are exactly the same, and if so, why. Within the limits of experimental error, these two properties appear to have exactly the same value, but do we know of any underlying physical reason that they should? ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 22, 2024, 11:02pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/32 "2024-12-22T23:02:19Z")
> [@hibernicus](#): > > these two properties appear to have exactly the same value, but do we know of any underlying physical reason that they should? It’s necessary to preserve the equivalence of gravity to acceleration in General Relativity. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 22, 2024, 11:06pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/33 "2024-12-22T23:06:59Z")
Well, yes, it’s one of the postulates of General Relativity. That’s why we care that it’s true. But is there any reason why it should be true? Of course, that’s partly a meaningless question, because even if we did say that some particular thing were the reason, we could just ask why that thing is what it is. ---
**Author:** ![3AxisCtrl](https://avatars.discourse-cdn.com/v4/letter/3/ecd19e/32.png) [@3AxisCtrl](https://boards.straightdope.com/u/3AxisCtrl)\ **Post date:** [December 22, 2024, 11:52pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/34 "2024-12-22T23:52:25Z")
> [@zimaane](#): > > Imagine that you were on Earth and there was a 1000 kg sphere suspended above ground on a rope attached to the ceiling. Could you brace yourself and push it away from you? Maybe a little, but it wouldn’t be easy. It would be quite easy to create a small horizontal displacement. If we assume the rope is 8’ long, a horizontal force of 10 pounds would displace the ball about half an inch. And it only stops there due to gravity. A better thought experiment would be to consider the way a massive object moves when a small force is applied and neither gravity nor static friction resist the motion. An example would be adjusting the position of a large boat that’s alongside a dock. Provided wind and current are not factors and you are (as you should be) content with slow motion, the force you must apply is surprisingly small - 20 lbs will easily suffice for a 20-ton boat. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 23, 2024, 6:15am UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/35 "2024-12-23T06:15:55Z")
A neat real-live example: > The tungsten weight is “only” 373 kg, but that’s still quite substantial. A few posters correctly estimated Don Pettit’s mass as 75 kg based on the relative displacement. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 23, 2024, 5:02pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/36 "2024-12-23T17:02:09Z")
Why the heck does the ISS have that much dead weight aboard; what’s it for? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 23, 2024, 5:14pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/37 "2024-12-23T17:14:41Z")
Well, they call it a “counterweight”. I imagine that they move it around to various places to make sure the station’s center of mass is where they want it to be. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 23, 2024, 5:19pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/38 "2024-12-23T17:19:33Z")
I could see that they’d want microgravity experiments to be as close to the center of mass as possible to minimize the Earth’s tidal pull. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 23, 2024, 5:52pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/39 "2024-12-23T17:52:09Z")
When they boost ISS to a higher orbit, they need to have the center of mass in a particular place. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 23, 2024, 9:08pm UTC](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558/40 "2024-12-23T21:08:41Z")
Somewhat disappointingly, it’s actually a counterweight for their exercise machine. You can’t tell from the vid, but it’s actually a stack of plates that’s been shipped up over a few flights. > **[NASA’s SpaceX CRS-29 Mission Overview - NASA](https://www.nasa.gov/international-space-station/commercial-resupply/spacex-crs-29-mission-overview/)** > > NASA and SpaceX are targeting 8:28 p.m. EST Thursday, Nov. 9, to launch the company’s 29th commercial resupply services mission to the International Space > - **Tungsten Plates** – The metal plates will serve as the counter mass, a weight that balances another weight, for the space station’s exercise device technology. The plates are designed to integrate with the E4D (European Enhanced Exploration Exercise Devices). A total of 14 plates will launch in two additional upcoming flights. > - **Tungsten Tray Lower Counter Mass** – Each tray will house two tungsten plates and will be assembled in orbit. The trays will stack together to become the lower counter mass for the station’s exercise device technology. > **[Overview for NASA’s Northrop Grumman 21st Commercial Resupply Mission - NASA](https://www.nasa.gov/general/overview-for-nasas-northrop-grumman-21st-commercial-resupply-mission/)** > > NASA, Northrop Grumman, and SpaceX are targeting no earlier than 11:29 a.m. EDT on Saturday, Aug. 3, for the next launch to deliver scientific investigations, > **Tungsten Plates** – A total of 14 tungsten plates will serve as the counter mass of the Vibration Isolation & Stabilization System designed to integrate with the European Enhanced Exercise Device. According to the design whitepaper: > **[VIS\_MSA\_Paper\_v6\_final.pdf](https://ntrs.nasa.gov/api/citations/20230000609/downloads/VIS_MSA_Paper_v6_final.pdf)** > > 34.07 MB > The purpose of a VIS system is to protect the on-orbit spacecraft from crew-imparted exercise loads through attenuation, and to provide sufficient platform stability for a crewmember to exercise safely and effectively. Kinda surprising that “crew-imparted exercise loads” could meaningfully affect the station, but I guess they’ve done the math and they need some attenuation system. [Previous page](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558.md?page=1) [Next page](https://boards.straightdope.com/t/about-space-and-gravity-too-long-to-explain-in-thread-title/1011558.md?page=3)