Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Reaction to centrifugal force

  1. Oct 10, 2008 #1
    Can anyone comment on the reaction force to the centrifugal force?
  2. jcsd
  3. Oct 10, 2008 #2


    User Avatar
    Homework Helper

  4. Oct 11, 2008 #3
    Take the case of a massive planet where we consider the force of gravity acting on a much smaller mass. There are two main possibilities:

    1) If the kinetic energy of the small mass is lower then some threshold, it will orbit around the planet.

    2) If the kinetic energy of the small mass is greater then some threshold, it will be scattered away from the planet by the centrifugal forces.

    In case 2, what happens to the planet?

    Answer: it is scattered away from the smaller mass ever so slightly by the exchange of some angular momentum with the smaller mass.
    Last edited: Oct 11, 2008
  5. Oct 11, 2008 #4

    D H

    Staff: Mentor

    The latter is very non-standard terminology. Anyone with even a very non-standard ax to grind can grind it quite to quite a fine edge on the internet, and wikipedia is one of the chosen places to grind those non-standard axes.

    It is best, in a forum widely read by students who already are in dire need of help understanding basic concepts to avoid introducing confusing and contradictory nomenclature.

    Apples and oranges. Reactive forces are very real. Fictitious forces aren't. There is, for example, no third law interaction (i.e., a reactive force) that makes Alpha Centauri appear to accelerate at 220 megameters/second2 when viewed by an Earth-fixed observer. That apparent acceleration results from a fictitious force.
  6. Oct 12, 2008 #5


    User Avatar
    Homework Helper

    It's common usage in English language. From my 1993 Webster's dictionary, "centrifugal force n: the force that tends to impel a thing or parts of a thing outward from a center of rotation". The current Webster's online defintion: "the apparent force that is felt by an object moving in a curved path that acts outwardly away from the center of rotation". Neither definition mentions anything about a frame of reference.

    Take the case of a rock on a string being twirled around in a circle. Centripetal force is the force the string applies to the rock, and centrifugal force is the apparent (reaction) force applied by the rock to the string. The statements hold true regardless of the frame of reference.
  7. Oct 12, 2008 #6

    D H

    Staff: Mentor

    That word apparent certainly does. Apparent force, fictitious force, inertial force, and pseudo force are synonyms. A quasar half way across the universe undergoes an apparent centrifugal acceleration when viewed by an Earth-fixed observer. There is obviously no real force involved here. The acceleration is purely a consequence of the observer's reference frame.

    Dictionaries also define weight as being synonymous with mass, which it is, legally. The discussion here is on terms as commonly used in physics. Centrifugal force is now almost universally used to connote a fictitious force rather than a real reactive force. Using the same term to describe two significantly different concepts leads to confusion, which is why physicists rarely (if ever) use the term "reactive centrifugal force" to refer to the 3rd law counterpart to centripetal force.

    Apparent forces are purely frame dependent. Reactive forces are very real and are frame independent.
  8. Oct 12, 2008 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Lots of common words have very different meanings in a physics context.
    Note that both definitions contradict your preferred usage! Using your definition, "centrifugal force" does not act on the object which goes in a circle, but both of these definitions imply that it does.

    In any case, please don't use Webster's as a physics reference. :rolleyes:

    Hey Jeff: Find us a reference in a current physics book that uses that archaic definition.

    By using such a definition, you are just adding to confusion.
  9. Oct 12, 2008 #8


    User Avatar
    Homework Helper

    So physics books get to redefine the English language? An how is it archaic if it's in current English dictionaries?

    I don't see any source of confusion here; why would either definition cause confusion? I would prefer a definition that is independent of the frame of reference, since forces are normally independent of frame of refence (outside of general relativity).

    Centrifugal force does act on an object that goes in a circle, the object that applies the centripetal force feels an apparent centrifugal force from the object that the centripetal force is being applied to. For example, if you're the outside person in an amusement park ride that whirls the riders around with no restraints and a slippery seat, the person inside you feels a centripetal force from you, and you feel a centrifugal force from the person inside you (and a centrepital force from the outside wall of the ride, so basically you feel compression). Both you and the person inside are moving in a circle. In the case of a rock on a string being twirled, the string end near the rock is moving in a circle.

    And this is a good thing? I would consider changing the meaning of a common word to be the source of confusion here. Why not invent a new term instead of redefining an existing term?
    Last edited: Oct 12, 2008
  10. Oct 12, 2008 #9

    D H

    Staff: Mentor

    Doc Al and I already showed how the dictionary definitions contradict your archaic definition.

    And yes, physics books do get to redefine the English language. To lay people and to the law, weight is a synonym for mass. The National Institutes of Standards and Technologies go so far as to acknowledge that physicists have a different concept of weight -- and ignore it. Weight is mass, legally. Physicists never think of weight as being mass. Weight has units of force to a physicist.

    Scientists and engineers often have to perform calculations in a rotating frame. I challenge you, for example, to come up with an inertial frame representation of the Earth's atmosphere. Non-inertial frames give rise to various pseudo forces, all of which have names. The apparent force resulting from the acceleration of the origin with respect to inertial: third body forces. The apparent force resulting from an object having a non-zero velocity in the rotating frame: Coriolis force. There's one more: The apparent force resulting from an object having a non-zero displacement from the origin of the rotating frame. You can call this whatever you want, but everyone else calls it centrifugal force.

    Yes, it does.
    It feels a very real, not apparent, reaction force. The standard name for that force, in the case of a rock on a string, is tension.

    The use of the term centrifugal force to connote the apparent force on an object that is displaced from the origin of a rotating frame is quite old. I don't know the etymology of your term; regardless, physicists do not use your definition anymore.
  11. Oct 12, 2008 #10


    User Avatar
    Homework Helper

    It's not my terminology, it's the terminology as defined in dictionaries. I think Wiki got it right by using a prefix to clarify, "reactive centrifugal force", which is essentially a different teminology than just "centrifugal force". Using "reactive" as a prefix is good enough for me.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Reaction to centrifugal force
  1. Centrifugal Force (Replies: 12)