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ballista force

My freind and I recently made a small ballista using hollow braid polypropalene rope in the skein. We got quite a bit of force out of it. My friend's dad (who is a cognative scientist and very smart) came to pick him up and looked at it. he said the rope wasn't storing very much energy and that it was the wood being pulled in by the rope. I think he's wrong. the wood doesn't move much(at least that's what I see) and i wanted someone else's opinion. in case you don't know what a ballista is, google it and youll find PLENTY of good sites.

here are the specs: the skein has four strands of polypropalene rope, skein goes from the center of a 6" piece of 2X6 to the center of the other

please help, i know i'm right!

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Reply to
Sir Shoelace
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I believe that the skein stores potential energy, just like a spring. The wood in the frame may deflect slightly, but would probably have the effect of dampening the spring.

I think your friend's dad is wrong. But I know what he is thinking. If you twist a rope while holding both ends, then pull the ends apart axially, you will get more accelleration in the unwind than if you just let it unwind without the axial pull.

However, in this design, with a minimum amount of twist in the skein ( came to pick him up and looked at it. he said the rope wasn't storing

Reply to
Kevin Alexeff

The intent of the Ballista design is to store all of the potential energy (prior to release) in the tension of the skein. In order to do this, the structure must be as solid as possible to support the heavy load that is built up when large forces are developed in the skein. Traditionally animal sinew was used to make the skein because of its ability to absorb large amounts of strain energy and quickly release it, just as it did when it was in the living animal. (Obviously it is not possible to store every last bit of energy in the skein; some small part will be stored in the deflection of the frame, but if the frame is very stiff, this part will be very small.)

I don't know enough about the material you used to say how good it is for energy storage, but it seems safe to say that in the Ballista design, whatever energy you were able to store was stored in the skein. (This assumes that you had enough strength in the frame that it was not deflecting significantly.)

Internal rubbing friction is a major cause of damping in an engine of this sort, and that is obviously something that needs to be minimized.

At the time of release, the potetial energy stored in the skein is converted into kinetic energy of the arms and the projectile, again assuming no stretch in the cord connecting the arms and no bending in the arms. Any stretching and bending will absorb some of the energy and slow the release to the projectile, not a desireable effect, so the no-stretch, non-bending criteria are the design goals.

Reply to
Dr. Sam

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