Re: pvc pipes and projectiles

[brian whatcott <inet@INTELLISYS.NET>]

At 16:14 10/12/01 -0400, Dwight Souder wrote:

> A while back, someone had written about a launcher
> they made using 2 pvc pipes
> and some surgical tubing (giant rubber band) to
> launch a golf ball.  One pvc
> pipe would fit inside and slide of the other pipe.
> The rubber band was attached
> to the bottom of both pipes (I believe).
> The golf ball would be loaded inside
> of it so that it rests ontop of the inner pipe,
> the inner pipe is pulled down,
> and the inner pipe is snapped upward launching the
> golf ball.  I've tried making
> some, but have had limited success.
> Has anyone tried making one?  Can anyone
> offer any suggestions?  I know it's difficult
> to explain, especially since I
> don't have a picture of it.
>
> Thanks,
> Dwight Souder
> Ashland, OH

Using only the description provided by Dwight, it seems that
the effort needed to cock the device rises from zero to
some maximal value.
If we modeled elastic bands as having a limiting extension
of 700% then the available energy is proportional to abt 3/7
of the final force times the inner pipe's length.
[Some initial extension is needed to straighten the elastic band
connected between the bases of concentric pipes]

It might be useful to improve that factor of 3/7.
One way might be to prestretch the driving elastic to 3.5
times its natural length, and fix it from the upper end of
the outer pipe to the lower end of the inner pipe, so
that the maximal draw force again stretches it to a
limiting 700%
Now, the available energy seems to have increased to 0.75
of the final draw force times the inner pipe's length.

[This note illustrates how a concretization of a question
somewhat similar to Ludwik's abstract mode 1/2 k v^2 spring
query works, and can be elaborated to discuss compound
bows...]


brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!