From: Steven Eiger (eiger@montana.edu)
Date: Thu Nov 14 2002 - 12:17:31 PST
Message-Id: <l03102802b9f9b23a7b82@[153.90.150.107]> Date: Thu, 14 Nov 2002 13:17:31 -0700 From: Steven Eiger <eiger@montana.edu> Subject: Re: Pinhole Digest #1059 - 11/14/02
I would say yes, but I think I am wrong. It reminds me of braking on a
bicycle. As you brake I always told people that the center of mass moves
froward. but can this be true? It seems to me that as the bike decelerates
it is because of a vector by the ground on the bike pushing against the
forward movement. This creates an angular force that creates a larger
downward force on the front wheel and a much smaller one on the rear wheel.
Thus if you use your back brake you are always in danger of skidding and
losing control, and the front brake decelerates you so well thaat your
inertia can carry you over the handlebars. Thus since the downward vectors
are so different on each wheel can we really say the center of gravity has
changed?, it seens more like centrifugal force, did we call those unreal or
virtual forces? I am in Montana now and I must say that one needs to learn
to ride very differntly here in the winter. Braking with the back brake is
suicidal. Eiger
> Center of Mass?
> by "Marc Afifi" <marc_afifi@yahoo.com>
> Re: pinhole Center of Mass?
> by <pauld@exploratorium.edu>
>
>
>----------------------------------------------------------------------
>
>Subject: Center of Mass?
>From: "Marc Afifi" <marc_afifi@yahoo.com>
>Date: Wed, 13 Nov 2002 15:25:32 -0800 (PST)
>
>I should konw the answer to this but I seem to be
>drawing a blank. I have a video demonstration of a
>cart sliding down an inclined plane first with its
>back wheels locked (cart slides down normally) and
>then with its front wheels locked (cart skids around
>until the locked wheels are in back). What's going on?
>Is this a center of mass phenomenon?
>
>-Marc
>
>__________________________________________________
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>----------------------------------------------------------------------
>
>Subject: Re: pinhole Center of Mass?
>From: <pauld@exploratorium.edu>
>Date: Wed, 13 Nov 2002 18:44:32 US/Pacific
>
>Hi Marc
>
>It is a center of mass and torque demonstration.
>
>Gravity acts downward through the center of mass.
>
>A normal force acts upward on each wheel perpendicular to the inclined plane.
>
>Friction forces act parallel to the surface and opposite the motion of the
>cart
>through the centers of the contact areas of the two locked wheels.
>
>Friction forces act on the rolling wheels but only in a direction parallel
>to the
>axle.
>
>These vector forces times their distance from any reference point gives a net
>torque on the cart.
>
>The torque twists the cart in the direction you describe.
>
>More explanation requires a diagram, want more?
>
>Paul D
>
>
>> I should konw the answer to this but I seem to be
>> drawing a blank. I have a video demonstration of a
>> cart sliding down an inclined plane first with its
>> back wheels locked (cart slides down normally) and
>> then with its front wheels locked (cart skids around
>> until the locked wheels are in back). What's going on?
>> Is this a center of mass phenomenon?
>>
>> -Marc
>>
>> __________________________________________________
>> Do you Yahoo!?
>> U2 on LAUNCH - Exclusive greatest hits videos
>> http://launch.yahoo.com/u2
>>
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Steven Eiger, Ph.D.
Department of Cell Biology and Neuroscience and the WWAMI Medical Education
Program
PO Box 173148
Montana State University - Bozeman
Bozeman, MT 59717-3148
Voice: (406) 994-5672
E-mail: eiger@montana.edu
FAX: (406) 994-7077
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