Date: Fri, 21 May 1999 07:20:18 -0700
From: Richard White <rwhite@telis.org>
To: Pinhole Listserv <pinhole@exploratorium.edu>
Subject: Re: More coins on free-fall rides
Many thanks to the folks that offered their explanations of why a coin
on a free-fall amusement park ride "floats", rather than freely-falling
with the car and rider. Most of the explanations were in line with the
inimitable Paul D's, invoking an "elastic-cantilever" action:
> Picture the ride as a bent spring cantalever.(Like the coin on the end of a
> hacksaw blade sticking out off the end of a table.)
> The hacksaw blade or the drop zone car is bent downward until the upward
> force produced by the bending counteracts the gravity force on the
> coin.(plus the weight of the car.)
> When the car is released it goes into freefall, in the freefall frame we
> ignore gravity.
> However the spring is still bent.
> The spring straightens out flinging the coin into the air in the freefall
> frame.
Although this seems a reasonable explanation to me intuitively, one
engineer/physics teacher postulated that the free-fall car is probably
not flexible enough to account for the coin's action. Instead, he
suggests that the center of mass of the car is somewhat offset
horizontally from the suspension point above the car. The tension force
from the suspending cable thus applies some torque about the center of
mass, causing the car to tilt slightly forward -- presumably, there is
some small amount of free play between the car and the rail that would
allow for such a tilt. When the car is released, the torque-producing
tension is eliminated, and the car rotates into it's more stable,
"upright" position almost immediately, causing the coin to accelerate
into the air (as seen from the car's reference frame).
Sounds plausible. Anyone able to debunk this possibile explanation?
(My students are dying to know!)
Richard White
Berkeley High School