# What is the difference between centrifugal force and gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [July 16, 2003, 12:36pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863 "2003-07-16T12:36:44Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [July 16, 2003, 12:36pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/1 "2003-07-16T12:36:44Z")
And if they are not the same, then what information do you need to determine which one is affecting you? ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [July 16, 2003, 12:40pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/2 "2003-07-16T12:40:24Z")
Per Einstein, there is no inherent way to tell the difference between gravity and acceleration. Centrifugal “Force” is a form of acceleration. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [July 16, 2003, 1:02pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/3 "2003-07-16T13:02:52Z")
Is gravity also a form of acceleration? ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [July 16, 2003, 1:50pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/4 "2003-07-16T13:50:49Z")
But centrifugal force varies with distance from the center, and so does gravity. If you’re on an orbiting space ship, there will be a very slight tidal force which would tend to cause your feet and your head to want to be in slightly different orbits. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [July 16, 2003, 1:57pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/5 "2003-07-16T13:57:34Z")
There is a famous thought experiment describing a person in a closed box, like an elevator car. If they drop an object and it falls downward with an acceleration of 1_g_, two things could be true: [ul][li]The elevator car is at rest on the Earth’s surface (or on some other body with a natural gravity of 1_g_, or[/li][li]The elevator car is accelerating through space at 1_g_.[/li][/ul] There is a flaw, though. If you drop a ball near the side of the elevator car, it won’t fall exactly straight (i.e. perfectly parallel to the car’s sides) if you are on Earth. Rather, it will fall toward Earth’s center of gravity, on a slight inward angle relative to the car. Thid wouldn’t happen if the car was moving with a uniform acceleration of 1_g_. The deviation will be slight, but postulating a large enough elevator car and precise enough instruments… Anyhoo, centrifugal force isn’t really a force as such. Rather, it’s your own momentum being blocked, which _feels_ like a force. Your weight wants to keep going in a straight line (as any object flung off a spinning base wants to) but this damn bulkhead keeps getting in the way. Clear as mud? ---
**Author:** ![Turek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/turek/32/18782_2.png) [@Turek](https://boards.straightdope.com/u/Turek)\ **Post date:** [July 16, 2003, 2:03pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/6 "2003-07-16T14:03:12Z")
The effect of gravity between two bodies is acceleration. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [July 16, 2003, 2:08pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/7 "2003-07-16T14:08:04Z")
You might find this guy’s whacky idea interesting. > [@](#): > > Can gravitation and inertia be identical? > This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a “real” gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. > > According to Newton, this interpretation is impossible, because in Newton’s theory there is no “real” field of the “Coriolis-field” type. But perhaps Newton’s law of field could be replaced by another that fits in with the field which holds with respect to a “rotating” system of co ordinates? My conviction of the identity of inertial and gravitational mass aroused within the feeling of absolute confidence in the correctness of this interpretation. > > Albert Einstein ---
**Author:** ![Dogface](https://avatars.discourse-cdn.com/v4/letter/d/9d8465/32.png) [@Dogface](https://boards.straightdope.com/u/Dogface)\ **Post date:** [July 16, 2003, 2:27pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/8 "2003-07-16T14:27:15Z")
Technically speaking, centrifugal force does not exist. That is, there is no “outward force” that is directed at a vector outwards from the center of a spinning system. What is actually happening is a combination of infinitesmals of force, every one tangental to the circular direction of spin. The vector sum of these infinitesmals at any instant is perceived as being a single force going outward in a single vector from the center. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [July 16, 2003, 2:41pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/9 "2003-07-16T14:41:11Z")
Is inertia another term for centrifugal force? ---
**Author:** ![Phage](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@Phage](https://boards.straightdope.com/u/Phage)\ **Post date:** [July 16, 2003, 3:02pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/10 "2003-07-16T15:02:56Z")
Umm… blocked inertia is centrifugal force. ---
**Author:** ![Liberal](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Liberal](https://boards.straightdope.com/u/Liberal)\ **Post date:** [July 16, 2003, 3:35pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/11 "2003-07-16T15:35:08Z")
Then did Einstein mean _blocked_ inertia in the passage cited by Ring? ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [July 16, 2003, 7:42pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/12 "2003-07-16T19:42:53Z")
> [@](#): > > _Originally posted by Bryan Ekers \* > \*\* > There is a flaw, though. If you drop a ball near the side of the elevator car, it won’t fall exactly straight (i.e. perfectly parallel to the car’s sides) if you are on Earth. Rather, it will fall toward Earth’s center of gravity, on a slight inward angle relative to the car. Thid wouldn’t happen if the car was moving with a uniform acceleration of 1_g\*. The deviation will be slight, but postulating a large enough elevator car and precise enough instruments… > \*\* This doesn’t help. If the ball falls at an angle due to the elevator wall, it is just as reasonable to assume that you are being accelerated in a direction that is not perfectly aligned with the elevator walls. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [July 16, 2003, 8:25pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/13 "2003-07-16T20:25:52Z")
Believe me, if you are building a rotating space habitat, you will soon find that there is a lot of difference between gravity and centrifugal force; in a small ship, or wheel, or habitat coriolis forces will be so strong that your inner ears will be affected, and you may well be unable to adapt; and thrown or falling objects will fall in a curving trajectory, making many activities difficult. This is not so apparent in a large rotating habitat. * * * SF worldbuilding at [http://www.orionsarm.com/main.html](http://www.orionsarm.com/main.html) ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [July 16, 2003, 8:27pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/14 "2003-07-16T20:27:36Z")
You can distinguish between the cases by dropping two balls. If their paths converge, you are in a gravitational field. If they diverge, you are spinning. If the paths are parallel you are accelerating (or you are too far from the center of mass/axis of rotation to measure the convergence/divergence of the paths). An object rotating about the origin with a constant velocity has an acceleration that is center directed. The magnitude of the acceleration is proportional to the distance from the center and to the angular velocity. Since you are accelerating toward the center, you experience a force away from the center.origin ---
**Author:** ![newcrasher](https://avatars.discourse-cdn.com/v4/letter/n/b3f665/32.png) [@newcrasher](https://boards.straightdope.com/u/newcrasher)\ **Post date:** [July 16, 2003, 8:30pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/15 "2003-07-16T20:30:36Z")
Gravity is green. Centrifugal force is blue. Remember G for gravity and G for green. I don’t have a trick for remembering centrifugal force. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [July 16, 2003, 10:11pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/16 "2003-07-16T22:11:19Z")
> [@](#): > > \*Originally posted by Libertarian \* > \*\*Then did Einstein mean _blocked_ inertia in the passage cited by Ring? \*\* Maybe these further quotes from Albert will clear it up. > [@](#): > > An observer who is sitting eccentrically on [a rotating] disc K’ is sensible of a force which acts outwards in a radial direction, and which would be interpreted as an effect of inertia (centrifugal force) by an observer who was at rest with respect to the original reference-body K. > > “But the observer on the disc may regard his disc as a reference-body which is “at rest”; on the basis of the general principle of relativity he is justified in doing this. The force acting on himself, and in fact on all other bodies which are at rest relative to the disc, he regards as the effect of a gravitational field.” > > Albert Einstein > > Underlining mine What Einstein is saying is that in a non-rotating frame centrifugal force is nothing more than the reaction force which arises from causing an inertial mass to follow a non geodesic path. However in the rotating frame it’s a real force albeit one that does not have an apparent source. ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [July 17, 2003, 12:08am UTC](https://boards.straightdope.com/t/what-is-the-difference-between-centrifugal-force-and-gravity/188863/17 "2003-07-17T00:08:41Z")
To expand a bit on **Ring** ’s last quotation, it was very common in Einstein’s own presentations of GR to start from the thought experiment of a rotating system of coordinates. Suppose you’re rotating. What does **special** relativity tell you about how you see of the space round about. Well, objects moving perpendicular to your line of sight are contracted, so you’d regard the circumference of circles about you as smaller than if you were stationary. But the radius remains the same. Space has become curved. Rotating and non-rotating observers thus disagree about the **geometry** of the space round about them. And since curved space is manifest as gravity, they disagree about the “forces” they see operating around them. In the 1916 _Annalen der Physik_ paper where he first lays out the full completed theory, he uses this argument at the start to motivate the fact that the theory will involve non-Euclidean geometry (it’s section 3; see the translation in _The Principle of Relativity_, Dover, 1952, p115-8), though as far as I can see he never mentions the connection to centrifugal forces. That is spelt out in the equivalent passage at the start of the discussion of GR in _The Meaning of Relativity_ (Methuen, 1922; Chapman and Hall, 1967, p59): > [@](#): > > But, according to the principle of equivalence, [the rotating system] may also be considered as a system at rest, with respect to which there is a gravitational field (field of centrifugal force, and force of Coriolis). We therefore arrive at the result: the gravitational field influences and even determines the metrical laws of the space-time continuum. To judge by a quotation from it in Pais’ _Subtle is the Lord …_ (Oxford, 1982, p243), there’s an even more explicit statement in an earlier 1914 paper: > [@](#): > > we have no means of distinguishing a “centrifugal force” from a gravitational field, [and therefore] we may consider the centrifugal force to be a gravitational field. In one of his Mr. Tompkins books, George Gamow developed the throught experiment in the form of observers on a roundabout in quite some detail on a non-technical level. Would be an excellent place to look for a good explanation.