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Kayak and Canoe Design Bulletin Board
Re: Kinetic Energy is a Moot Point
Posted By: Nolan In Response To: Kinetic Energy is a Moot Point (Mike Scarborough)
Date: Friday, 27 August 1999, at 7:36 a.m.
> The thing about kinetic energy is that the only way to recover it is to
> reduce the velocity. (We’ll assume that our heavy kayak, paddler and load
> are one, unchanging, mass.)And, the only way to increase the kinetic energy of this unit is via more enthusiastic paddling to increase the velocity.
> But in a frictionless world, what would be the effect of additional mass?
> If the mass is constant and the velocity is constant the kinetic energy
> would also remain constant. If we added mass, there would be more kinetic
> energy, but the energy would be stored when we started paddling and
> accelerated the boat for the first time. In a frictionless world it would
> take no enery to propel a boat, regardless of mass, so long as the
> velocity remains constant.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.
> When our paddle is not in the water friction is
> slowing us down. (The work we do with our paddle is speeding us up.)Actually, water friction is slowing you down at all times of motion.
> has. Additional mass does not require us to work harder to maintain a
> constant velocity, except for the effect of increased draft, wetted area,
> and drag--which do not increase as fast as mass. This is because draft
> increases as the cube root of displacement, which is exactly equal to
> mass.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.
> If we were propelled by a propellor instead of a paddle we might assume
> that our velocity was constant. In that case there would be no
> acceleration/decceleration cycle and all our work would go into overcoming
> friction with no changes in kinetic energy.Which you can promptly verify as having a flaw by attempting to move a free rolling boxcar with a powerboat.
> Kinetic energy is a red herring!
Kinetic energy, while many things, is never a red herring. It's a fundamental.
> What a can of worms!!!
Aye, but physics is phun! :-)
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