Hello,
I'm not much of a wine drinker, so I wasn't familiar with this. But I
got out one of my wife's wineglasses and experimented with it. Indeed,
if the base of the glass is upon the table and I push it around in a
clockwise circle, the glass itself will strongly tend to rotate
counterclockwise.
However, if I move it similarly, but in hand and not touching the
table, no such counter-rotation occurs. The torque must be coming from
its contact with the table's surface.
So what's happening? Consider your experience inside a car, as it
traces a circular curve, for instance a cloverleaf ramp on the
freeway. As you do so the car, with you in it, stays on the circular
path. The central ("centripetal") force which bends the car's path
from straight (inertial) motion to circular is provided by friction
between the tires & the road pavement. If this centriptetal force were
too weak, the car's inertia would play a stronger role and the path
would be a more gradual curve which diverges from that of the road.
It's like an orbiting satellite, for which its horizontal speed is
balanced with the radial force of gravity, bending its path into a
circle.
So what else might you notice from within your car as it negotiates
the circular road? Well of course, you feel as if you're being pushed
outward. This is due to your own bodily inertia. If you watch other
cars on the curve, you may notice that, as they follow curve to the
right, they tilt to the left. The car body floats on a suspension
system connecting it to the wheels. The wheels are rather firmly on
the pavement, but the body has some limited freedom to pursue its
inertial motion, and it tilts. In fact, the whole car tends to tilt,
so the pressure between tire & road is greater for the outer set of
wheels than for the inner ones.
This is what happens to the wine glass as you slide it around on a
circle. It tends to tilt outward, and the edge of the base which is
more outward, at a greater radius, has more frictional force between
it & the table. This drag imparts torque on the edge of the base,
inducing a rotation. But as you can see, the direction of this torque
is necessarily in the exact opposite direction from that of the large
circular motion of the whole glass. |
