At 01:35 p.m. 25/07/02 -0400, Coplan, Karl wrote: >Ron Dunnington writes: > >"A rocket engine does not push against anything >except the rocket body and that is why it can operate with, or without an >atmosphere, or, even in water. A paddle must have something to push against >(more like a jet engine than a rocket), to be effective. " > >That's sort of my point. Rocket engines work on Newtonian principles -- >very light particles of rocket exhaust accelerated to extreme speeds >generates Newtonian thrust without anything to push "against." I agree. Let me phrase it in more Newtonian Terms. The rocket does not need any atmosphere to push against. The Newtonian momentum conservation principle states that if there are no external forces, then the rocket should maintain its momentum. Internal combustion is not an external force. So if particles move back very quickly due to combustion, the rocket must move forward a bit so that the center of mass of the system particles-rocket maintains its momentum. If large amounts of combustion particles come out fast, the rocket gains speed in the opposite direction but the center of mass stays as it was. On the other hand paddling is like sticking a pole in the mud and pulling ourselves from it. There is an external force, or reference that the system uses to pull itself from. For an ideally perfect paddle , it would enter the water and stay right there. Then all the power would be transferred to move the kayak forward, against that fixed pole. Paddles are not ideal and if they were would be too difficult to use when starting to accelerate. Since paddles leak some water and suffer some displacement from the entrance point, some energy is lost, but also may help the paddler if he doesn´t want to put all his strength or if he wants to save some energy in long tours. Stronger paddlers will want better grip in the water. Weaker fast stroking paddlers will want some leaking to allow them to give more strokes with less force, even at the expense of some energy loss. Best Regards, Rafael el cayuco chief *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Thu Jul 25 2002 - 18:16:34 PDT
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