Re: [Phys-l] Centrifugal redux; muddied

[curtis osterhoudt <flutzpah@yahoo.com>]

1) Have the students draw the velocity vectors of an object in uniform circular motion, at perhaps four or five swiftly successive times (whatever "swiftly successive" means in this context; perhaps so that the object goes through no more than pi/2 radians during the period of interest. If the students think the speed is not roughly the same at all times, that's a trickier proposition. This exercise still works, but somehow it's less convincing---too many links to go bad in the chain of reasoning---when one has to draw successive position vectors, then calculate velocities from those, then the accelerations. 
    Of course, it works for an object going in a zig-zag, too. 
2) From each successive pair of velocities, draw the acceleration vector (or at least its direction, or at least v2 - v1). In every case, the acceleration will point to the center of the circle. If the students accept N2, then there's really no indication of a centrifugal force on the object whatsoever.
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If students are still convinced that there is a centrifugal force somewhere ("Hey, the rope really is being held taut by *something*"), then _perhaps_ having them think about the same situation (massive object tied to the end of a rope, the other end of the rope being held by some prime mover) for the case of uniform linear acceleration might help. Car (or spaceship, if gravity gets in the way) accelerates in a straight line, with something (can; astronaut) tied to its bumper. Rope is actually held taut by the inertia of the object (or by N3, if one reasons that way), but in no way is there a force tugging the rope in the opposite direction of movement from the car (or spaceship). It is the car (or spaceship) wanting to get away from the _object_. 
   It also may help (I'm not sure why, but it does, in my experience) to have the students think about two massively disparate objects (an experiment can be done with a ping-pong ball tied to a bowling ball, if one can find a really frictionless lazy-susan on which to place the bowling ball), with one in orbit about the other. Keep changing the relative masses, so that the "orbiter" eventually becomes the "orbited". Keep asking the students to identify (to themselves) where the centrifugal force goes/comes from. In every case, there is easily-identifiable centripetal force, and it explains _all_ of the behavior of each system. 
   I wish I knew why that exercise seems to help some students see there isn't really a centrifugal force to be found (so long as inertia is a concept they're comfortable with), but I don't. Maybe someone on the list could clarify what processes might be going on. 





----- Original Message ----- 
From: "Stuart Leinoff" <leinoffs@sunyacc.edu>
To: "Phys-L List server" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, March 20, 2009 1:46 AM
Subject: Re: [Phys-l] Centrifugal redux; muddied


> ..and perhaps just to muddy the waters here a bit:
>
> Doesn't "centrifugal" just mean "away from the center" (or from the Latin
> "fleeing" the center)?
>
> Even in the laboratory (inertial) frame, if there is a centripetal force, 
> by N3
> there would have to be a centrifugal force.  (or are we all assuming that 
> we
> are talking only about forces acting on the object moving in a circle?)
>
> Even so, why would anyone argue that there is no such thing as centrifugal
> force?
> --
> Stuart Leinoff