Message-Id: <l03110736b360b2041d2c@[192.174.2.173]>
In-Reply-To: <373A6541.2693588C@telis.org>
Date: Thu, 13 May 1999 08:55:28 -0800
To: "Pinhole Listserv" <pinhole@exploratorium.edu>
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Re: Behavior of coins on free-fall rides
Hi Richard
It's nice to hear of someone else who rode the edge with a peanut butter
jar and a coin! I did too!
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.
If people are uncomfortable with the description in the accelerating frame
of reference given above, I will describe this in a frame of reference at
rest with respect to the earth.
At rest at the top of the ride the coin is pushed up by the car and pulled
down by gravity. Net force 0.
The car bends down under gravity down and an upward retoring force due to
bending. (Call it a spring force.) The net force on the car is also 0. The
spring force up equals the weight of the car (the gravity force down.)
When the car is released it accelerates down in freefall. Gravity provides
the force driving this downward acceleration. The upward spring force still
exists however because the spring of the car is still bent (compressed) So
the spring pushes up on things resting on the car (the coin) with a force
of 1 g at first This means that as the center of mass of the car drops at
g, the coin is pulled down by gravity and pushed up by the spring force of
the bent car at also 1 g so it doesn't start to accelerate with the car, it
simply stays behind at rest. As the car unbends, the upward force on the
coin rapidly decreases to zero. And the coin falls at g.
Paul D
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld