There seems to be some misunderstanding here about the physics of how things move. Hopefully I can explain some of this without boring or insulting anyone. Rockets and jets and cars and people's feet all push (exert force) on _something_. In the case of rockets and jets the something is the gases that are ejected to the rear. Extremely high pressures are created inside the rocke/jet engine and gases can only escape from the rear so there is resulting force towards the front that drives the vehicle. A kids balloon is a rocket vehicle. Air inside is under pressure, when the neck is closed the pressure/force in all directions is the same and the balloon sits there. When it's untied, air can escape and there is a net force in the direction opposite that of the escaping gases. Same in Jets. In jets and rockets tremendous pressures are generated by heating gases. In jets some of the gas is sucked in the front. In rockets all of the gases are the result of burning something. If the exit is blocked and pressure gets higher than the strength of the walls of the combustion chamber, they explode. When operating properly, the resultant force is proportional to both the velocity imparted to the material exhausted and mass of the material exhausted. The water in a jet ski exhaust doesn't have to move as fast to create an identical force as the gases in the exhause of a rocket motor have to move, because the water is denser than the gas. Propeller planes work because the propeller accelerates air towards the rear of the plane, boat propellers the same, acellerating water. Tires and feet accelerate the world and result in drive of the vehicle/ person in the other direction. In the last two cases if the friction is low (ice?) the force exerted on the world is low and the forward drive is low. The mass of the world is so large that the effect of the forces of tires and feet is generally negligible. The fact that tires and feet are generally exerting forces in different random directions tends to cancel out these relatively negligible forces, also. With kayaks, forward force is equal to the product of the acceration of the water times the mass of the water moved. If there is a lot of slippage (small area paddle blade = small mass of water) the paddle has to move backwards faster to get the same forward force, as you get with a large area blade (large mass of water) and slower movement of the water/ blade. If you move the same paddle slowly, you get less resultant force than if you move the paddle faster, as the force generated is proportional to speed achieved (acceleration). If you move a teeny paddle blade and a huge paddle blade at the same speed in the water you get more force to move the kayak from the huge blade. The force comes, originally from your body, so moving the small blade, at a given speed, requires less energy from you than large blade, moved at the same speed. Another example of the same principles of physics is a person throwing a baseball and a shot put. The strength of the person is constant, the baseball goes faster than the shotput because it is lighter (less mass). If identical energy is expended to throw them, they will each contain the same energy and injure you the same, if they hit you. A Honda del Sol going 75 mph and weighing 1 Ton (75 ton/mph), will do the same damage as a loaded 18 wheeler going 5 mph and weighing 15 Tons (75 ton/mph). Hope I helped somebody, and didn't confuse too many, Fairwinds and happy bytes, -- ------------------------------------------------------------------------- Dave Flory, San Jose, CA. daflory_at_pacbell.net Go Sea Kayaking!! ------------------------------------------------------------------------- Speak softly and study Aikido, then you won't need a big stick. ------------------------------------------------------------------------- *************************************************************************** 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 - 10:55:54 PDT
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