# If the Earth was a perfectly round ball of glass, and I dropped a ball on it, would it roll? **URL:** \ **Category:** Factual Questions\ **Created:** [September 8, 2016, 5:50pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350 "2016-09-08T17:50:58Z")\ **Posts on this page:** 20\ **Page:** 2
**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:** [September 8, 2016, 8:42pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/21 "2016-09-08T20:42:20Z")
Yes, I mistakenly typed “poleward” in that second paragraph. :smack: The net force due to centripetal force and therefore acceleration and therefore motion is equatorward, not poleward. Thanks y’all for the corrections. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [September 8, 2016, 8:42pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/22 "2016-09-08T20:42:47Z")
> [@sbunny8](#): > > It seems to me that, since Earth is rotating, the Coriolis force would cause the ball to roll towards the equator. Consider the extreme case where you placed the ball six inches away from the North pole. If the ball were glued to that spot, it would trace a six-inch-radius circle every 23 hours and 56 minutes. But without the glue, momentum would carry it away from the pole and it would roll towards the equator. But then, by conservation of angular momentum, it would drift towards the West. Ignoring wind resistance, it would eventually end up at the equator, rolling West at very high speed (from Earth’s point of view) Eventually by this it would also be rolling around the equator tidally locked to the sun, well the moon, some interacting between the 2, or something like that, even Jupiter would be a factor. ---
**Author:** ![Sasquatch\_1](https://avatars.discourse-cdn.com/v4/letter/s/8491ac/32.png) [@Sasquatch\_1](https://boards.straightdope.com/u/Sasquatch_1)\ **Post date:** [September 8, 2016, 8:54pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/23 "2016-09-08T20:54:36Z")
> [@Omar\_Little](#): > > Assuming it is perfectly round sphere; there was not friction; either from the earth or your ball; as long as you gave your ball some momentum; yes it should roll. Actually, if it really were frictionless, the ball would just slide, no reason for it to roll. I suppose the slight torque across the body of the ball from the Coriolis force would cause it to rotate, so it should end up sliding in a spiral down to the equator while building up rotation, then cross to the southern hemisphere and begin slowing down, starting a repeating cycle. Any other minor forces that would show up in the pure physics spherical cow world model? I bet solar/lunar tides would have some effect on the path, at least over the long term. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [September 8, 2016, 9:06pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/24 "2016-09-08T21:06:32Z")
The earth’s core is made up primarily of iron; that is more gravitationally dense than glass. Would a solid glass earth have enouth gravity to even keep the glass ball (or the observer) on the surface? ---
**Author:** ![Atamasama](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atamasama/32/12961_2.png) [@Atamasama](https://boards.straightdope.com/u/Atamasama)\ **Post date:** [September 8, 2016, 9:25pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/25 "2016-09-08T21:25:39Z")
Wouldn’t any outside force need to overcome the coefficient of friction for the glass? Glass is smooth but not frictionless. (Unless that’s what “perfectly smooth” is supposed to mean.) ---
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [September 8, 2016, 9:44pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/26 "2016-09-08T21:44:51Z")
Guys, I used “glass” to abbreviate the thread title. The idea is if the Earth were perfectly smooth surface area. What it’s made of is not relevant to the discussion. My fault. Poor articulation on my part. 🙂 ---
**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:** [September 8, 2016, 10:19pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/27 "2016-09-08T22:19:54Z")
> [@](#): > > Quoth **[t-bonham@scc.net](mailto:t-bonham@scc.net)**: > > Would a solid glass earth have enouth gravity to even keep the glass ball (or the observer) on the surface? What force do you imagine would be stronger than the gravity from glass-Earth, to pull it away? ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [September 8, 2016, 10:46pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/28 "2016-09-08T22:46:31Z")
> [@Chronos](#): > > Only if it’s magic glass. The rigidity of rock isn’t enough to prevent the Earth from assuming the “correct” shape (except on very small scales like mountains), and glass is less rigid than rock. The OP specified it was perfectly spherical, which would imply _some_ extra rigidity unless the extra several miles of thickness were dumped simultaneously and I don’t know anything about strain and stress equations to know what would happen then. Plus there’s the OP specification of no friction which implies magical properties to begin with 🆒 ---
**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:** [September 8, 2016, 11:06pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/29 "2016-09-08T23:06:51Z")
> [@Grrr](#): > > Yes, I guess I should state the sphere would be in a vacuum (No atmosphere) but still spinning on its axis around the sun. I was thinking if I delicately place it on the sphere though, as opposed to giving it momentum. I’m curious about this part here. If the earth is spinning (without an atmosphere) and you set a non-spinning ball on it, wouldn’t there be a moment (at touchdown) where the ball would suddenly start spinning wildly. Or does the OP assume that the ball is orbiting around the sphere at a matching speed at the moment of contact? ---
**Author:** ![Nature\_s\_Call](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nature_s_call/32/19006_2.png) [@Nature\_s\_Call](https://boards.straightdope.com/u/Nature_s_Call)\ **Post date:** [September 8, 2016, 11:10pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/30 "2016-09-08T23:10:38Z")
Fascinating discussion from a seemingly simple question. What _I_ really want to know, though: would a glass airplane on a glass treadmill take off 🙂 ---
**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:** [September 9, 2016, 12:36am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/31 "2016-09-09T00:36:46Z")
It would. But since it’d be transparent you wouldn’t see it. So you won’t believe it really happened. 😃 ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [September 9, 2016, 12:39am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/32 "2016-09-09T00:39:32Z")
> [@White SIFL](#): > > I’m curious about this part here. If the earth is spinning (without an atmosphere) and you set a non-spinning ball on it, wouldn’t there be a moment (at touchdown) where the ball would suddenly start spinning wildly. Or does the OP assume that the ball is orbiting around the sphere at a matching speed at the moment of contact? This would depend on the about of friction. At first the magical sphere would just be sliding underneath the ball, and the ball would begin to rotate (and start accelerating in the direction of the sphere’s spin). A little bit of friction, the ball would slowly spin more, a lot of friction and the ball will spin up faster. You can test this at your grandparent’s house if they have something called a “record player”. Turn it on and set a marble somewhere on the plastic disk … see how the marble starts to rotate. Also, the marble is slung off just like of ball on the sphere would slide (or roll) to the equator. We’ve been assuming the ball is placed on the surface such that the ball would appear stationary to someone standing on the sphere next to the ball, not exactly an orbit except at the equator. ETA: Those who fly glass airplanes should avoid birds … ---
**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:** [September 9, 2016, 12:44am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/33 "2016-09-09T00:44:47Z")
> [@White SIFL](#): > > I’m curious about this part here. If the earth is spinning (without an atmosphere) and you set a non-spinning ball on it, wouldn’t there be a moment (at touchdown) where the ball would suddenly start spinning wildly. Or does the OP assume that the ball is orbiting around the sphere at a matching speed at the moment of contact? The OP says to “place” the ball on the surface. The implication I get is that it’s set down with zero velocity relative to the surface. If instead it was set down with zero velocity relative to the Earth’s center and a fixed external reference frame then things are different. The ball will immediately have velocity westward relative to the surface. How fast depends on the cosine of the latitude. About 1000 mph at the equator and declining to zero at the poles. But since the OP specified the surface is frictionless, there will be no tendency for the small ball to spin. It’ll sit there with the same orientation to the fixed external reference frame while the planet rotates under it. ---
**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:** [September 9, 2016, 1:01am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/34 "2016-09-09T01:01:47Z")
> [@t-bonham@scc.net](#): > > The earth’s core is made up primarily of iron; that is more gravitationally dense than glass. > Would a solid glass earth have enouth gravity to even keep the glass ball (or the observer) on the surface? The Earth’s average density is about 5.5x that of water. Glass density varies by type, but is about 3x. So very roughly speaking, an Earth made of glass would mass about half what the real one does. This is ignoring whatever sort of weird glass would form under the tremendous pressures at the center; I’m just assuming it’s ordinary glass through and through. Given half the mass and the same size as real Earth, it’ll produce 1/2 the surface gravity. That’s plenty to not float away. Although as \*\*Chronos \*\*said, things don’t float away. They must be pushed or pulled away. By something. The OP’s magical problem doesn’t contain any extra somethings. ---
**Author:** ![Atamasama](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atamasama/32/12961_2.png) [@Atamasama](https://boards.straightdope.com/u/Atamasama)\ **Post date:** [September 9, 2016, 1:54am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/35 "2016-09-09T01:54:44Z")
> [@LSLGuy](#): > > It would. > > But since it’d be transparent you wouldn’t see it. So you won’t believe it really happened. 😃 You’d still see Wonder Woman suspended in midair in the invisible cockpit though. ---
**Author:** ![Nature\_s\_Call](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nature_s_call/32/19006_2.png) [@Nature\_s\_Call](https://boards.straightdope.com/u/Nature_s_Call)\ **Post date:** [September 9, 2016, 5:12am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/36 "2016-09-09T05:12:24Z")
> [@Atamasama](#): > > You’d still see Wonder Woman suspended in midair in the invisible cockpit though. The invisible man is sore, though… [Superman-WonderWoman Joke [rec.humor.funny]](http://www.netfunny.com/rhf/jokes/88old/supes.44.html) ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [September 9, 2016, 11:58am UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/37 "2016-09-09T11:58:33Z")
On a perfectly frictionless surface, once it starts “rolling” due to Coriolis, it would appear to be very slowly turning in the opposite direction of its motion compared to the surface under it. (Maintaining its orientation.) Would look a bit odd. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [September 9, 2016, 12:06pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/38 "2016-09-09T12:06:33Z")
> [@Grrr](#): > > Guys, I used “glass” to abbreviate the thread title. The idea is if the Earth were perfectly smooth surface area. What it’s made of is not relevant to the discussion. Well with that being the only difference, in that you take away all things earth (rock,water,air… ) , and rotation of planets being normal, since the glass spherical earth is rotating then the centrifugal force is throwing your marble out to the equator. Also since the moon and planets are there , and there is no friction or atmosphere, the moon and planets cause perturbations of the gravity which roll your ball around,even if the ball is perfectly on the equator. Normally the perturbations are too small compared to deformations (the flat bottom of a tyre is large enough for you to see … ), slope, wind and starting conditions (momentum that you impart … ) but for your no friction no atmosphere no deformations, perfect case… the moon and planets become important. Not sure what that would do to the glass, but the first thing to happen is the ball rolls… ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [September 9, 2016, 2:24pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/39 "2016-09-09T14:24:17Z")
> [@ftg](#): > > On a perfectly frictionless surface, once it starts “rolling” due to Coriolis, it would appear to be very slowly turning in the opposite direction of its motion compared to the surface under it. (Maintaining its orientation.) Would look a bit odd. The ball would appear to have another force acting on it pointed toward the West as it descends in latitude, and then pointed to the East as it ascended in latitude. The ball itself does not change speed, rather it is the point of the magical sphere underneath the ball that is accelerating and decelerating. ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [September 13, 2016, 3:08pm UTC](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350/40 "2016-09-13T15:08:46Z")
> [@Ignotus](#): > > The ball is, from a gravitational point of view, no different from a liquid. The tidal force on the ball, which would ultimately make it move, is the exact same force that would, in a confined space, make the level of a liquid rise in the same direction. Who said anything about making it “rise”? [Tidal forces are mostly horizontal, not vertical.](https://www.youtube.com/watch?v=pwChk4S99i4) It’s not about trying to make the liquid “rise”, it’s about making the liquid slosh sideways, east and west. What prevents this from happening in a teacup is NOT the small size of the liquid. It’s the fact that the cup has walls and the walls are prevented from moving. If you put a glass ball on a smooth surface or a cup of tea on a frictionless surface, either one of them would DEFINITELY move east and west with the tides. Yes I know that neither of them would move up and down but that’s irrelevant. They would move horizontally. [Previous page](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350.md?page=1) [Next page](https://boards.straightdope.com/t/if-the-earth-was-a-perfectly-round-ball-of-glass-and-i-dropped-a-ball-on-it-would-it-roll/765350.md?page=3)