Re: gravitational sensations--was Work/Energy theorem ?

["Richard W. Tarara" <rtarara@saintmarys.edu>]

Actually you never 'feel' the gravitational force, even when in free fall. 
Standing on the earth you only feel the upward push of the ground.  All of
our sensations of our own 'weight' are not sensations of the gravitational
force but rather of those due to the forces that prevent our weight from
accelerating us.  This 'backwards' interpretation of forces is very common
and ultimately responsible for the 'sensational' feelings we have on many
carnival rides.

Take away the upward forces (again free fall) and we feel
'weightless'--e.g. the shuttle astronauts are being pulled towards the
earth with about 90% of the their surface 'weight', but the absence of
upward (radially outward) forces creates the weightless sensations. 
Likewise we 'experience' the forward push of the airplane seat during
takeoff as a backwards push holding us in our seat.  The carnival rotor
ride seems to glue us to the outside wall while the only radial force is
that wall pushing us inward.  Analysis of a Ferris Wheel will also show
that we experience all the forces 'backwards'.

Now let's move to a situation without the gravitational force (or major
distortions in time/space {sorry but the GR explanation of gravitational
force isn't going to work in introductory classes where students are still
struggling with the concept of acceleration after 14 weeks or work  ;-( 
}).  If we now are subjected to JUST the 'upwards' force (say the floor of
a spaceship accelerating us--or a rotating space station whose outer wall
is pushing us towards the center of rotation) we 'feel' as though we have
'weight' and feel as though we are being held down.  

While one can get much more technical about all this, I thing for
introductory students the important point here is to stress that our
experience of forces is very often 'backwards' and this leads to some
interesting experiences in very special frames of reference.  

rick
----------
> > 	You may feel the force of gravity while standing on the surface of
> > the earth.  I, on the other hand, feel my body in perfect equilibrium
> > owing to the balance between the gravitational force and the
> > electromagnetic forces that give by bones structure....  In other words,
I
> > don't feel myself being accelerated....(ok I will admit to a very small
> > centripetal acceleration owing to the earth's rotation.)
> >
> > Mark