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Kayak and Canoe Design Bulletin Board
Re: Kinetic Energy is a Moot Point
Posted By: Nick Schade In Response To: Re: Kinetic Energy is a Moot Point (Nolan)
Date: Friday, 27 August 1999, at 11:13 a.m.
> And, the only way to increase the kinetic energy of this unit is via more
> enthusiastic paddling to increase the velocity.We do not need to increase kinetic energy, just maintain it. The only thing robbing the energy is drag.
> No, adding mass does not increase the kinetic energy in your example. The
> kinetic energy will stay the same, and velocity will decrease. You can
> verify this yourself with an old turntable or lazy susan by spinning it,
> and then dropping on a bag of flour. Rotational speed will immediately
> decrease. If you carefully knock the bag of flour back off, rotational
> speed will immediately increase. If you don't like the rotational aspect,
> use an office chair with wheels.You can not make the turn table go faster by knocking off the bag of flour. The bag will continue to spin after you knock it off, retaining its energy. The turntable will continue at its slow speed.
> Actually, water friction is slowing you down at all times of motion.
> You are assuming you as a paddler have an infinite amount of energy to
> impart onto a mass, this is not true. This can in fact be observed in the
> frictionless environment of space by watching objects the astronauts move.
> Small low mass objects are able to be propelled at a greater velocity then
> larger objects. Why? In part the F=MA, so they are not able to accelerate
> a large object as well as a small one. And also in part because they are
> limited in their own strength and can only impart a limited energy
> onto/into the body. Hence, if KE is limited, M and V are inversely
> proportional.It is not a true statement that low mass objects are able to be propelled at a greater velocity than high mass objects. Given enough energy, you can make any object move at any speed. But I know what you are thinking of. When a low mass object is expelled from a high mass oject the low mass moves faster. This is conservation of momentum (mass * velocity). The momentum of the two ojects must add up to their original momentum. For two objects touching at rest, when they are pushed apart the momentum of one will equal the momentum of the other. If one object has half the mass of the other, it needs to have twice the velocity for momentum to be conserved. Note that the lower mass (higher velocity) object will have higher kinetic energy (KE = m*v^2) because the velocity component is squared.
> Which you can promptly verify as having a flaw by attempting to move a
> free rolling boxcar with a powerboat.> Kinetic energy, while many things, is never a red herring. It's a
> fundamental.> Aye, but physics is phun! :-)
Ain't it tho
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