# Centripetal force question **URL:** \ **Category:** Factual Questions\ **Created:** [January 9, 2012, 6:40am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899 "2012-01-09T06:40:57Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![DaveBfd](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DaveBfd](https://boards.straightdope.com/u/DaveBfd)\ **Post date:** [January 9, 2012, 6:40am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/1 "2012-01-09T06:40:57Z")
I would like to hear a simplified explanation of centripetal force. I’ve looked up a few links and they haven’t been helpful to me. Either it is too late at night for this or I have simply failed to grasp this concept. Can somebody wiser explain it for me? Under what circumstances do we measure this force, how does it act? ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [January 9, 2012, 7:07am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/2 "2012-01-09T07:07:49Z")
When an object travels in a circular path, **centripetal force** is the name we give to the force that causes the object to travel in that circular path, as opposed to a straight line (that is, in the absence of any other forces). Note that centripetal force acts _inward_, along the radius of the circular path. For an object being swung around on a string, the magnitude of centripetal force is equal to the tension on the string. For an object in orbit around the Earth, the magnitude of the centripetal force is equal to the force of gravity acting on the object. When you drive around a sharp curve to the left in a car, centripetal force accelerates you to the left (i.e. inward). You may think there is a force acting on you to the right (i.e. outward), but that is only true in the reference frame of the car itself: this fictitious force in the car’s non-inertial reference frame is known as _centrifugal force_. Centrifugal force is equal in magnitude to centripetal force, but opposite in direction. P.S. Yes, I’ve seen this [xkcd strip](http://xkcd.com/123/). However, in my experience of teaching this topic to introductory physics students, I find they have enough trouble learning Newton’s laws in an inertial frame, rather than complicating the issue with non-inertial rotating frames. I have no problem with referring to centrifugal force as a “fictitious force.” ---
**Author:** ![Hyperelastic](https://avatars.discourse-cdn.com/v4/letter/h/a3d4f5/32.png) [@Hyperelastic](https://boards.straightdope.com/u/Hyperelastic)\ **Post date:** [January 9, 2012, 2:15pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/3 "2012-01-09T14:15:30Z")
It takes a force to keep you moving in a circular path, even if your speed is constant. Otherwise you’d shoot off in a straight line tangent to the circle. That force is the centripetal force. You can’t measure a force directly; its magnitude is determined by the laws of mechanics and the observed motions or deformations. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 9, 2012, 2:29pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/4 "2012-01-09T14:29:42Z")
> [@robby](#): > > For an object in orbit around the Earth, the magnitude of the centripetal force is equal to the force of gravity acting on the object. Feynman had an interesting comment apropos of this (paraphrased): “People used to think planets moved because angels pushed them along. It turns out there are angels - it’s just that they push inward.” ---
**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:** [January 9, 2012, 5:14pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/5 "2012-01-09T17:14:35Z")
> [@](#): > > For an object being swung around on a string, the magnitude of centripetal force is equal to the tension on the string. This is true, but it doesn’t go far enough, and might therefore be misleading. Not only are those two forces equal, they’re _the same force_. When an object is being swung around on a string, the tension in the string _is_ the centripetal force. As for centrifugal force, you can call it a “fictitious force” if you absolutely must, just so long as you make it clear that this is just terminology, and that you don’t mean it doesn’t actually exist. A better term would be “inertial force”. Centrifugal force is exactly as real as gravity, and I’m willing to bet you tell students that gravity really exists. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [January 9, 2012, 5:57pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/6 "2012-01-09T17:57:51Z")
> [@Chronos](#): > > This is true, but it doesn’t go far enough, and might therefore be misleading. Not only are those two forces equal, they’re _the same force_. When an object is being swung around on a string, the tension in the string _is_ the centripetal force. A very good point. I should have added just such a statement. > [@Chronos](#): > > As for centrifugal force, you can call it a “fictitious force” if you absolutely must, just so long as you make it clear that this is just terminology, and that you don’t mean it doesn’t actually exist. A better term would be “inertial force”. Centrifugal force is exactly as real as gravity, and I’m willing to bet you tell students that gravity really exists. We’ve have this discussion before, as I recall. 😃 (Where does the time go? I joined the SDMB back when I was teaching physics, and it’s now been nearly a decade since I last taught.) Anyway, one problem with bringing centrifugal force into the mix is that students may decide that both centripetal force and centrifugal force are both acting on an object traveling in a circular path (in an inertial reference frame), and therefore come to the erroneous conclusion that the net force on the object is zero! I think it is more helpful to teach introductory physics students that centrifugal force is a fictitious force and that it does in fact **not** exist, so long as it is emphasized that we are talking about an inertial reference frame. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [January 9, 2012, 6:06pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/7 "2012-01-09T18:06:07Z")
> [@DaveBfd](#): > > I would like to hear a simplified explanation of centripetal force. It’s the force that can be delivered by 100 legs acting in unison. Oh, wait…never mind; I thought you wrote “centipedal”. ---
**Author:** ![DaveBfd](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DaveBfd](https://boards.straightdope.com/u/DaveBfd)\ **Post date:** [January 9, 2012, 10:04pm UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/8 "2012-01-09T22:04:37Z")
Thanks for the replies, it turns out I understood it right but was expecting something else. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 10, 2012, 12:35am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/9 "2012-01-10T00:35:53Z")
Does the centrifugal force depend in any way on the mass in/of the surrounding Universe? Or is that a question of philosophy and not physics? ---
**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:** [January 10, 2012, 12:48am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/10 "2012-01-10T00:48:19Z")
There is some speculation that the mass of the rest of the Universe is needed in order to have something for the rotation to be relative to, but I don’t think there’s any way to prove or disprove that. Certainly you can’t use centrifugal force to determine anything about the mass of the rest of the Universe. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [January 10, 2012, 2:18am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/11 "2012-01-10T02:18:34Z")
> [@KarlGauss](#): > > Does the centrifugal force depend in any way on the mass in/of the surrounding Universe? Or is that a question of philosophy and not physics? I don’t see how. (I’m an engineer, not a physicist nor a philosopher.) F=ma. The only mass involved is the mass of the body being accelerated. Even if the rest of the universe didn’t exist (no external reference frame) and it was just you in a spaceship, you could still tell that you were rotating by measuring the resulting force, i.e. artificial gravity. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 10, 2012, 2:57am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/12 "2012-01-10T02:57:33Z")
> [@standingwave](#): > > Even if the rest of the universe didn’t exist (no external reference frame) and it was just you in a spaceship, you could still tell that you were rotating by measuring the resulting force, i.e. artificial gravity. I hesitate to respond because I really don’t have a good sense for this issue but will stay say, ‘not so fast there’. I don’t think it’s a given that you’d observe the effects you’ve come to expect (e.g. artificial gravity) if there was nothing else in the Universe (to rotate with respect with). Which way would the artificial gravity be directed in the absence of anything other than the ship? How could there be a single, preferred direction in such a situation? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [January 10, 2012, 3:50am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/13 "2012-01-10T03:50:00Z")
> [@KarlGauss](#): > > Does the centrifugal force depend in any way on the mass in/of the surrounding Universe? Or is that a question of philosophy and not physics? This is discussed a bit in [this thread](http://boards.straightdope.com/sdmb/showthread.php?t=631599). In post 47, **EdwardLost** claims that it does\*, although he doesn’t seem to have a cite. I’m still wavering on that one. - ETA: not directly, but I think that has to follow. Follow the link and read what he actually wrote. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [January 10, 2012, 4:10am UTC](https://boards.straightdope.com/t/centripetal-force-question/608899/14 "2012-01-10T04:10:25Z")
> [@ZenBeam](#): > > This is discussed a bit in [this thread](http://boards.straightdope.com/sdmb/showthread.php?t=631599). In post 47, **EdwardLost** claims that it does\*, although he doesn’t seem to have a cite. I’m still wavering on that one. > > - ETA: not directly, but I think that has to follow. Follow the link and read what he actually wrote. Thanks. His Mach’s Principle link (in post #47 as above) took me to [Newton’s Bucket Argument](http://en.wikipedia.org/wiki/Bucket_argument) which, in retrospect, is what I must have been thinking of.