I have other concerns with the gormless Fc
. Textbook authors don’t really seem to use it and its namesake, the “centripetal force”, honestly. The adjective “centripetal” is a valuable label when describing a familiar force that has a component responsible for keeping an object moving in a circle (my definition). For example: “When Mitt Romney makes a 180° turn in policy, political expediency is the centripetal force”. Traditional texts define the centripetal force as Fc ≡ Fnet
, which is a problem for non-uniform circular motion. But even if we understand that they really mean the radial component of the net force, authors seldom use it this way in their descriptions; they tend to describe single, inward forces as the centripetal force. Furthermore, this term and its notation seem to be no more than an affectation of high school and introductory physics texts. In more advanced studies the term is abandoned and central forces are described or angular motion and moments are used.
Unfortunately, students are often quite relieved when you provide them with a new force, Fc
. Perhaps it provides a convenient scapegoat on which to blame the mystery of circular motion. Just as novice physics students will often invent a mysterious new force to explain the upward acceleration of a held object (see the FBD above), they are happy to have a brand new force to explain the peculiarities of circular motion. Perhaps you have noticed how Fc
tends to appear in free-body diagrams in curious locations or in the place of other, reasonable forces. They will stop thinking carefully about how friction might be keeping the car going around the corner. Why should they: it’s the Fc
that’s responsible! Other times the Fc
appearing in their FBDs corresponds to no known physical interaction. But what can we expect: neither does their physical experience of circular motion! (The outward force, that is.)