# Gravity **URL:** \ **Category:** Factual Questions\ **Created:** [August 28, 2017, 7:59am UTC](https://boards.straightdope.com/t/gravity/794912 "2017-08-28T07:59:31Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![abashed](https://avatars.discourse-cdn.com/v4/letter/a/c6cbf5/32.png) [@abashed](https://boards.straightdope.com/u/abashed)\ **Post date:** [August 28, 2017, 1:03pm UTC](https://boards.straightdope.com/t/gravity/794912/21 "2017-08-28T13:03:33Z")
> [@abashed](#): > > That was not explained, so I much appreciate your fuller explanation. I have learnt something new. Thank you. 🙂 No, it wasn’t that actual video but he said much the same thing. 🙂 ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [August 28, 2017, 1:04pm UTC](https://boards.straightdope.com/t/gravity/794912/22 "2017-08-28T13:04:15Z")
Gravity sucks … > [@abashed](#): > > [snip] … Yes and I guess this must be due to the warping of each other’s space, that I do know about but then is this the answer, i.e. the geometry of spacetime? I’m getting awfully:confused: confused. What the kind folks here have been talking about is Newtonian gravity, which is close enough to work in considering human-sized concerns, and most everything that happens on Earth … there’s a few phenomena in the solar system that requires using Einstein’s gravity and General Relativity which is where time/space comes into play … Newton treats gravity as an innate property of matter … it just is … and Newton’s Law of Gravity works well enough until speeds get close to that of light … and the fastest man-made object was the Juno probe at her first closest approach to Jupiter, a little less than a thousandths of the speed of light … ---
**Author:** ![abashed](https://avatars.discourse-cdn.com/v4/letter/a/c6cbf5/32.png) [@abashed](https://boards.straightdope.com/u/abashed)\ **Post date:** [August 28, 2017, 1:04pm UTC](https://boards.straightdope.com/t/gravity/794912/23 "2017-08-28T13:04:46Z")
> [@chrisk](#): > > I’m not going to get into the cause of gravity… but should say that gravity is not an acceleration. It is a force proportional to mass. This force will either be countered by some equal and opposite force, or it will cause acceleration. > > When we’re standing on a solid surface, the “down” force of gravity is countered by an upwards force of the solid surface supporting us. That creates the usual sensation of weight. When you’re falling, there’s very little force to counter gravity, and you’re accelerating downward. (Air resistance will counter a little.) Thanks chrisk. 🙂 ---
**Author:** ![abashed](https://avatars.discourse-cdn.com/v4/letter/a/c6cbf5/32.png) [@abashed](https://boards.straightdope.com/u/abashed)\ **Post date:** [August 28, 2017, 1:05pm UTC](https://boards.straightdope.com/t/gravity/794912/24 "2017-08-28T13:05:53Z")
> [@Machine\_Elf](#): > > Actually, you’re weightless as soon as you jump. No matter which direction you’re going, if gravity is the only force acting on you, then you are “weightless.” Got it Elf. 😉 ---
**Author:** ![abashed](https://avatars.discourse-cdn.com/v4/letter/a/c6cbf5/32.png) [@abashed](https://boards.straightdope.com/u/abashed)\ **Post date:** [August 28, 2017, 1:07pm UTC](https://boards.straightdope.com/t/gravity/794912/25 "2017-08-28T13:07:43Z")
> [@watchwolf49](#): > > Gravity sucks … > > What the kind folks here have been talking about is Newtonian gravity, which is close enough to work in considering human-sized concerns, and most everything that happens on Earth … there’s a few phenomena in the solar system that requires using Einstein’s gravity and General Relativity which is where time/space comes into play … > > Newton treats gravity as an innate property of matter … it just is … and Newton’s Law of Gravity works well enough until speeds get close to that of light … and the fastest man-made object was the Juno probe at her first closest approach to Jupiter, a little less than a thousandths of the speed of light … Yeah, that word ‘relativity’ again. ☹ ---
**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:** [August 28, 2017, 1:11pm UTC](https://boards.straightdope.com/t/gravity/794912/26 "2017-08-28T13:11:42Z")
The key is not that the ground is moving towards the falling person. It might or might not be, depending on the reference frame you use and the particular moment you measure. The ground is, however, accelerating upwards. You can prove this simply enough, by just taking an accelerometer and placing it on the ground: It’ll read one g. It’s doing that all the time, whether you’re in contact with it or not. How is it doing that everywhere? Well, that’s where curvature of spacetime comes in, and it gets too complicated for words on a message board… ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [August 28, 2017, 1:20pm UTC](https://boards.straightdope.com/t/gravity/794912/27 "2017-08-28T13:20:02Z")
> [@Little\_Nemo](#): > > It’s even possible to have enough air resistance that it is stronger than gravity and prevents the object from falling down. Leaving out the perverse case of the air moving upward, how would that work? ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [August 28, 2017, 1:30pm UTC](https://boards.straightdope.com/t/gravity/794912/28 "2017-08-28T13:30:20Z")
> [@abashed](#): > > When someone jumps off a tall building why are they free from gravity until they hit the ground? This is going to get complicated… When Einstein saw that very event happening the eventual result, which fully explains the physics, is the General Theory of Relativity. So, let the explanations fly! ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [August 28, 2017, 1:36pm UTC](https://boards.straightdope.com/t/gravity/794912/29 "2017-08-28T13:36:07Z")
> [@Wallaby](#): > > The container _ **and the water** _ are both in freefall. This means that any water that escapes through the sideholes (it would be significantly less in free fall than when stationary, since the water will have no weight, and therefore exert no pressure) would stay level with the container. > > However, I expect the water may appear to travel upwards relative to the container because air resistance would affect individual water drops more than the container. (It would be slight, but probably noticeable). That reminds me of when we were kids and one of our favorite shows was Sky King. My friend was positive that once you pulled the ripcord and your chute opened you would start rising. This of course was due to the effect caused by the cameraman continuing to fall after the actors chutes opened. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 28, 2017, 1:52pm UTC](https://boards.straightdope.com/t/gravity/794912/30 "2017-08-28T13:52:03Z")
> [@Xema](#): > > Leaving out the perverse case of the air moving upward, how would that work? I presume that fine enough dust or nanoparticles could take an arbitrarily long time to settle to the ground. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 28, 2017, 1:52pm UTC](https://boards.straightdope.com/t/gravity/794912/31 "2017-08-28T13:52:22Z")
> [@abashed](#): > > … > If I were to jump off a building standing on a weighing machine, although I would be in freefall there would still be a little force acting on the weighing machine because there would be a tiny bit of gravitational attraction between me and the machine. When I reached the ground (discounting the impact) the resistance of the ground would stop my fall and I would experience my full weight. Is this right? > … You need to be a bit careful here. Basic physics is properly done in a no gravity no atmosphere no friction environment. In a place like that you will see the full beauty of simple equations come into play. _A ha!_ moments are easy and are likely to be correct. Anywhere else that has a gravity well, an atmosphere, or friction you need to correct for those confounding variables to see the basic physics pop out at you. Here’s how this applies to the bit I quoted above: If you jumped off a building with a scale on your feet _and there was no atmosphere_ then: You and the scale would be accelerated towards the Earth and rapidly gain speed towards the Earth. And the Earth would microscopically gain speed towards you. The Earth wouldn’t move even the width of an electron, but Nature’s books stay balanced as you both work to pull each other together. Meanwhile, as you say, because both you and the scale attract one another the scale will read something other than zero. But that value will be in picograms. It is not zero, but it’s very, very close to zero. Now let’s add an atmosphere back into the story. The Earth will still attract you and you it. But your acceleration will be significantly slowed by air drag. So the neat clean formulas of basic physics will not apply exactly. The farther you fall the more this is true. But the difference is immediately measureable by instruments and fully appreciable to ordinary human senses even falling just 20 feet. The scale will still attract you and you it. But your acceleration versus the scale will be significantly affected by air drag. So the neat clean formulas of basic physics will not apply exactly. The farther you fall the more this is true. But the difference is appreciable to unaided human senses even falling just a couple feet. If the scale has more air drag than you do, the scale may show that you “weigh” many kilograms. OTOH, if you are draggier than the scale, it will fall away from your feet. And it’ll read zero since you and it aren’t touching. In other words, you “weigh” less than zero from the scale’s POV since you’re getting farther apart. When air drag is measured in kilograms and the person-scale attraction is measured in picograms, it’s easy to get confused about what’s really going on. In _principle_, the person attracts the scale and vice versa. In _practice_ the air drag utterly overwhelms this effect in the first microseconds of fall and continues to do so until impact with the ground. Bottom line: the effect of air drag is gigantic compared to gravity unless you’ve got something the size of a planet doing the pulling. Whenever you watch physics on Youtube (which IMO is usually a way to learn garbage rather than true stuff) you need to always correct what you see for gravity, friction, and atmosphere. Because the vids that show something unexpected tend to be popular, there’s a strong incentive for vid producers to play some sleight of hand with those factors. As others have said, the bit with the spraying water tank in free fall n an atmosphere is exactly that kind of sleight of hand. All the “surprise” comes from air drag. Something we all deal with every second of every day and therefore learn to sorta ignore and sorta expect depending on context. When placed in an unfamiliar context we can easily get the ignore/expect thing backwards… ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [August 28, 2017, 2:00pm UTC](https://boards.straightdope.com/t/gravity/794912/32 "2017-08-28T14:00:35Z")
> [@Little\_Nemo](#): > > …It’s even possible to have enough air resistance that it is stronger than gravity and prevents the object from falling down… > [@Xema](#): > > Leaving out the perverse case of the air moving upward, how would that work? It’s not air _resistance_ (aerodynamic drag, which by definition will only oppose the direction of motion), but for an object less dense than air [buoyancy](https://en.wikipedia.org/wiki/Buoyancy) will propel it upwards. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [August 28, 2017, 3:43pm UTC](https://boards.straightdope.com/t/gravity/794912/33 "2017-08-28T15:43:02Z")
> [@bob\_2](#): > > It’s easy to become weightless. Just jump into the air - at the point where you stop going up and start coming back down - you are weightless. Actually, as soon as your feet leave the ground you are weightless. All the way up and down. If your eyes are closed, you cannot tell you changed direction (assuming no air resistance). We used this in our Zero Gravity Facility at Glenn Research Center. We had a large accelerator at the bottom of the 500 foot deep bore and could propel experiments upwards (in a vacuum) and they dropped back down. This gave double the experiment duration compared to the usual drop. There is no difference to the experiment as it falls upwards or down. Dennis ---
**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:** [August 28, 2017, 3:54pm UTC](https://boards.straightdope.com/t/gravity/794912/34 "2017-08-28T15:54:56Z")
Huh, I didn’t know that the drop-pit at Glenn had upward launching capability-- Nobody mentioned it when I toured there. Was that removed at some point? In any event, though, the physics point is correct. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [August 28, 2017, 4:13pm UTC](https://boards.straightdope.com/t/gravity/794912/35 "2017-08-28T16:13:53Z")
Let’s go to the Zero Gravity Facility at Glenn Research Center and throw stuff down there! ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 28, 2017, 4:35pm UTC](https://boards.straightdope.com/t/gravity/794912/36 "2017-08-28T16:35:41Z")
You mean throw stuff up there. 😃 ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [August 28, 2017, 5:49pm UTC](https://boards.straightdope.com/t/gravity/794912/37 "2017-08-28T17:49:49Z")
> [@LSLGuy](#): > > You mean throw stuff up there. 😃 For sure – tooling around in zero gravity would likely make me throw up. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [August 28, 2017, 6:18pm UTC](https://boards.straightdope.com/t/gravity/794912/38 "2017-08-28T18:18:31Z")
> [@abashed](#): > > [snip] … If I were to jump off a building … [snap] > [@LSLGuy](#): > > You need to be a bit careful here … [snap] Ya think? Nominated for understatement of the week … ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [August 28, 2017, 6:48pm UTC](https://boards.straightdope.com/t/gravity/794912/39 "2017-08-28T18:48:29Z")
You’re going to have a throw-down by throwing-up? Oh oh… > [@LSLGuy](#): > > All the “surprise” comes from air drag. Something we all deal with every second of every day and therefore learn to sorta ignore and sorta expect depending on context. Somehow, I think you deal with this just a bit more than the rest of us Earth-bound plebians. 😉 ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 28, 2017, 7:56pm UTC](https://boards.straightdope.com/t/gravity/794912/40 "2017-08-28T19:56:06Z")
> [@RitterSport](#): > > For sure – tooling around in zero gravity would likely make me throw up. There is a reason NASA’s plane that simulates zero-g is nicknamed the [Vomit Comet](https://www.nasa.gov/vision/space/preparingtravel/kc135onfinal.html). If you like for a cool $5200 you can book space on a [Zero-G plane](https://www.gozerog.com/) near Las Vegas. They basically allow paying passengers to do what previously only astronauts got to experience on the Vomit Comet. [Previous page](https://boards.straightdope.com/t/gravity/794912.md?page=1) [Next page](https://boards.straightdope.com/t/gravity/794912.md?page=3)