Re: Centrifugal force

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

The alternative way to talk about the sensations in the rotor ride or in
other accelerating frames is to consider your sensation of weight as you sit
in a stationary chair or stand on a floor.  We have come to interpret the
upward force of the chair or the floor as being the sensation of weight.
Therefore, when the chair is pulled out or we step off the roof of a
building, our sensation of weightlessness is because we have removed the
force that we normally perceive as weight.  [Here weight is defined as the
earth's gravitational pull on one--not the alternative and equally valid
'what the scale reads' definition.]

In other words, we have come to learn to experience many forces
'backwards'--hence our belief in the centrifugal force in the rotor ride.
We can even create the sensation of weight by having the 'upwards' force in
the absence of the downward force of gravity (accelerating space
ship--rotating space station).  In this mode, orbital motion is NOT
weightless, only apparently so since the pull of the earth is still about
90% what it would be on the surface, but because of the constant falling of
the orbital ship, there are no 'upwards' forces to give us the sensation of
weight.  [Hecht, for one, follows this line.]

IMHO, this is not a bad way to go with an introductory class, especially a
liberal arts type class.  It keeps one firmly set in the inertial frame
viewpoint and preserves Newton for all these motions.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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----- Original Message -----
From: "Joseph Bellina" <jbellina@saintmarys.edu>
> So what is the centrifugal force.  Think of the fairground ride called
> the rotor.  You get in, it starts spinning and the floor falls away, but
> you stay held against the wall.  In the earth frame, the only horizontal
> force acting on you is the horizontal component of the normal force by
> the wall (the wall may not be completely vertical.)  This is true for the
> same reasons I described above.
>   In your accelerating frame you feel an outward force pushing you against
> the wall.  That is the centrifugal force.  It only exists in the
> accelerating frame.  A similar situation existed in an accelerating car.
> When the car is accelerating forward, what force pushes you back in the
> seat? You feel pushed back but that is your sensation of the forward force
> by the seat on you.  If the driver slams on the brakes, what force pushes
> you into the windshield, or hopefully into the seat belt...there is none,
> you continue to move forward (N1) until the windshield or the seat belt
> slows you motion.