At 8:17 PM -0500 7/25/02, Rafael Mier Maza wrote: >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. > The mistake is to think that just because you are on the earth you have something to push against. The earth is floating through space just like the rocket. The rocket pushes a small mass out behind it to make it move. A person on the earth just has a much larger mass to push out behind. The physics is the same. The real difference is in perception. On a rocket the frame of reference is usually the rocket and not the mass you are pushing out behind. This makes it look like the mass of propellant does most of the moving. On the earth the frame of reference is usually the earth so it looks like the person does all moving. For the purposes of analysis you can usually ignore the motion of the earth even though the principles remain the same. However when you are on the water you are not all that strongly bound to the structure of the earth. When you try to push off of it by dipping your paddle in the water, you are dipping into a fluid that can move independently from the earth. As a result you need to treat paddling more like a rocket when you analyze it. -- Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 (860) 659-8847 *************************************************************************** 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 Fri Jul 26 2002 - 06:27:00 PDT
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