Centrifugal VS Centripetal

  1. Hi all.
    This is not a homework question, I'm 33 and far away from homework. It is just a personal doubt. I've read a lot about these forces, even some answers from these forums but still can't make something out of them.

    What I've read:
    *Centripetal: a force that makes objects attracted to a rotating center.
    *Centrifugal: a pseudo-force that makes objects go away from a rotating center.

    This is what confuses me: in a merry-go-round the faster you gyrate, the greater the force to push you away from it (Centrifugal force). I have observed that personally when I was a kid. But the Centripetal definition states that it should be the other way around, and it isn't. So, if the earth is rotating at a high speed, why aren't we thrown out of it?

    -Gabriel.
     
  2. jcsd
  3. Nathanael

    Nathanael 1,132
    Homework Helper

    The way I imagine it is that the centrifugal force is essentially caused by your inertia (which is trying to move you in a tangential straight line).
    (I think this is why it's not a "real" force, it's just an "apparent force" that exists in a rotating perspective.)
    The centripetal force is also greater. It's just that from the perspective of someone rotating on a merry-go-round, you only notice the centrifugal force.

    Gravity provides more than the needed centripetal force to keep us rotating with it. (The excess force going towards our weight)
     
    Last edited: Jul 6, 2014
  4. Thanks Nathanael! What I get from your reply is that if it wasn't for gravity, we would in fact be thrown out of the planet. So Earth's gravity is far greater than the Centrifugal force the Earth spin has on us.

    I was under the idea that gravity was a product of centripetal forces, but it seems that is not the case.

    So, we know the gravity constant for our planet, but, what causes it?
     
  5. stevendaryl

    stevendaryl 2,988
    Science Advisor

    Centripetal is not a particular kind of force, it's just a word to mean any force directed toward the center. So gravity is the centripetal force for the moon orbiting around the earth. But if you have a rock tied to the end of a string, and you're rotating it around, then the tension in the string is the centripetal force. If there isn't a centripetal force, then the object won't go in circles, it will fly off in a straight line.

    Centrifugal force is not actually a force. It's an apparent, or "fictitious" force that seems to be pulling an object away from the center when the object is traveling in a circular path. In actuality, the object doesn't have anything pulling it away from the center. If you are twirling a rock on the end of a string, and you suddenly let go, the rock isn't going to fly radially outward; it's going to continue traveling in the same direction it was going, along the straight-line tangent to the circle.
     
  6. Nathanael

    Nathanael 1,132
    Homework Helper

    If the earth were not spinning at all, it would still attract us.

    What causes gravity? That question is way beyond me. I'm not even sure if it's understood very well (I think that it's not, but perhaps it is).

    I haven't studied General Relativity (Einstein's theory of gravity) but if you're interested in gravity, that would be the thing to study.
     
  7. A.T.

    A.T. 5,621
    Gold Member

    What exactly did you observe? The outwards acceleration in the rotating frame of reference is what the Centrifugal pseudo-force is accounting for.

    If the Earth was rotating at high enough speed to throw us out, it would fall apart itself or rather had never formed a planet.
     
  8. The merry-go-round applies a force on you to make you move in a circle. If it didn't you would move in a straight line. The force provided by the merry-go-round is a centripetal force and it acts towards the centre.

    Newton third law (summarised on Wikipedia) states "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body." That equal and opposite force we call the centrifugal force.

    On the surface Earths gravity provides a lot more centripetal force than is required to move in a circle (a circle that has a radius equal to that of the earth at one revolution per day). To prove it if you jump off a cliff you don't stay at the same radius. Instead gravity pulls you towards the centre of the earth tightening your radius.

    The rotation of the earth does have some effect. If you weigh yourself on scales you find you weigh less at the equator than at the poles. If I remember correctly the difference is only a few % because gravity is much stronger than necessary to keep you attached to the earths rotating surface.
     
  9. A.T.

    A.T. 5,621
    Gold Member

    The OP is explicitly asking about the centrifugal pseudo-force, which exists in rotating frames and is not subject to Newtons 3rd Law.

    What you describe is a real-force acting on the second object, that provides the centripetal force. Even though this force can act "outwards", the consensus among mentors on PF is to avoid the "centrifugal" label, to avoid confusion with the pseudo-force described above.
     
  10. NascentOxygen

    Staff: Mentor

    The only way I can imagine anyone being thrown off the Earth by natural means would be if you were unlucky enough to be sitting atop a volcano when it decided to pop its top---big time.
     
  11. LOL one of the funniest posts in physicsforums xD.

    Theoretically if there wasnt any gravity, still the force of friction could ve keep us on earth unless ofcourse the earth was rotating with very highspeed v such that mv^2/R>max friction. Ahhh but wait, no gravity no friction also...
     
  12. Nathanael

    Nathanael 1,132
    Homework Helper

    But friction isn't centripetal? Wouldn't friction only accelerate you tangentially? (assuming horizontal surface and ignoring the fact that, as you said, no gravity means no friction)

    So it could theoretically keep you moving with earth briefly, but the ground would soon "curve away" from under your feet

    That's how it seems to me at least
     
  13. Yes you are right nathanael, friction at the surface of the earth can be only tangential to the surface.
     
  14. sophiecentaur

    sophiecentaur 13,566
    Science Advisor
    Gold Member

    I think the main reason for the trouble that people have with the fugal / petal thing is that we do not directly experience either centrifugal force or gravity. What we actually do feel is the reaction due these forces on ourselves - through our feet or our body in a seat. We misinterpret the 'being thrown outwards' sensation and the pressure of our body as being the true description of what's happening. To get a better understanding, it seems to be necessary to chuck out the intuitive view and start from the 'pure' direction.
     
  15. Is it really any different from what you experience undergoing linear acceleration? When a car accelerates you feel as if you are being pressed back into the seat when in reality there isn't anything pulling you backwards - perhaps we should we also be calling that a pseudo force? Oh wait..
     
  16. WannabeNewton

    WannabeNewton 5,767
    Science Advisor

    Under what circumstances are you talking about?

    Friction can certainly be centripetal depending on the physical system e.g. a bug moving around in a circle on a rough plane will obviously be subject to a friction that is centripetal.
     
  17. sophiecentaur

    sophiecentaur 13,566
    Science Advisor
    Gold Member

    I wouldn't disagree with that. However, we can be irrational and there are many other clues to tell us that the car is accelerating and we have 'learned' to cope with that situation, perhaps. Having said that, 'everyone' feels they are being 'thrown forward' when the brakes are applied. That's another situation in which we have to get formal and not go by instinct if we want to get it right.
     
  18. As you know, the centrifugal force is a by-product of the spinning motion of the merry-go-round. You are sitting in the merry-go-round, so you experience this outward (pseudo) force.

    Viewed from the outside world, you are moving in a circular motion. Due to inertia, you have a tendency at each moment to continue in a straight line, which would mean flying off the merry-go-round. You don't fly off the merry-go-round because you're sitting on a horsey (or, if it's one of those self-powered carousels, you may be hanging on for dear life). The horsey (or your hanging on) is providing the inward centripetal force that keeps your path on a circle.

    Simply put, the force of earth's gravity is much stronger than the centrifugal force (which is just how we, in our "earth" frame of reference, experience our tendency to continue in a straight line).
     
  19. rcgldr

    rcgldr 7,462
    Homework Helper

    To clarify this, fictitious centrifugal force is a caculated value of angular velocity and radius, ω^2 * r and is the apparent force as observed from within a rotating frame on any object, including an object outside the rotating frame. If the object is not moving with respect to the center of the rotating frame, then to an observer within the rotating frame, that object experiences an outwards psuedo centrifugal force = ω^2 * r and an inwards coriollis psuedo force of 2 ω^2 * r resuting in an apparent centripetal force of ω^2 * r.

    If the observed object is at rest with respect to the rotating frame, then the apparent centrifugal force is opposed by some real centripetal force, the forces "cancel", and the object is not accelerating with respect to the rotating frame.

    Fictitious centrifugal force could be considered to be similar to artificial gravity.
     
    Last edited: Jul 9, 2014
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