Re: Some More Problems

[Jim Riley <jriley@lib.drury.edu>]

A similar problem which is easier to set up is to take a spool of wire with
end disks bigger than the roll of wire, place it on the desk so that the
wire is unwinding from the bottom of the spool and ask the class which way
the spool will roll when you pull on it.  The answer depends on the angle
the wire makes with the horizontal. 

At 04:17 PM 2/15/98 -0800, you wrote:
> Those are good problems, Herb.
>
> A few years ago at an AAPT meeting held at the University of
> British Columbia I perfomed the demonstration of pulling backwards
> on the lower pedal of a bicycle. I used a very low gear (24") and
> extended the crank. The direction of motion (acceleration) of the
> bicycle depends upon where one pulls along the crank. One can pull
> backwards and have the bicycle move forward!
>
> The most interesting thing is that I consider that result to be
> counterintuitive; many in the audience did not. They were somewhat
> perplexed when asked to explain why I could get different result
> by pulling at different points along the crank. I wrote up that
> demonstration, but I can't find it now. I'll post it if I do. One
> must have a suitably low-geared touring bike to do it. I use the
> bike I ride up to school on. I climb more than 300 meters from my
> home, so I've got granny gears on it. Most "mountain bikes" have
> extra low gears on them nowadays. I have a mountain bike, but I
> ride a derailleur touring bike to school when I ride.
>
> Leigh
Jim Riley
Department of Physics
Drury College
Springfield Missouri  65802
(417) 873 7233
e-mail:  jriley@lib.drury.edu
fax: (417) 873 7432