The Kayak Forum is Sponsored By
|
|
|
| |
Kayak and Canoe Design Bulletin Board
Kinetic Energy is a Moot Point
Posted By: Mike Scarborough In Response To: Re: Dynamic versus static weight (Nolan)
Date: Thursday, 26 August 1999, at 5:18 p.m.
I got up this morning and it was pouring rain, putting the kibosh on my plans to spend the day outside repairing kayaks. Then I got into this thread, and realized that I was fated to spend the morning writing about mass and kinetic energy. Then I got to Nick’s post, which is spot on. Unfortunantly I don’t have a physics text with me and my memory after twenty years is far from perfect, so maybe some of our physicist friends can fill in the missing equations. I think the part I’m missing relates to power and work.
KINETIC ENERGY, FRICTION, MASS, and ACCELERATION
An object at rest has no kinetic energy. An object in motion does and it is related to it’s mass and it’s velocity. (And I’ll assume that Nolan’s equation, KE=.5MV^2, is correct)The object got it’s velocity because something applied a force to it for a given length of time, causing it to accelerate. (F=MA) Kinetic energy is the energy STORED in an object by virtue of it’s mass and velocity.
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.)
We all seem to agree that increased mass increases draft, which increases wetted area, which increases drag.
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.
Now, we don’t live in a frictionless world. When we paddle our velocity is not constant. It peaks near the end of our stroke and reaches a minimum as we start a new stroke. When our paddle is not in the water friction is slowing us down. (The work we do with our paddle is speeding us up.) Friction reduces our velocity by applying a negative acceleration (A=F/M). Our kinetic energy is reduced. Where does it go? The water and the hull are slightly heated by the friction. But note that mass is now the denominator in the equation. The amount we slow down between paddle strokes is less the more mass our boat has. (Friction remaining constant.) All we have to do is supply enough power to overcome friction. On each stroke friction slows us down and we paddle to speed us back up. The force of friction equals the force we apply with our paddle. Our kinetic energy fluctuates during this cycle, but it flucuates less the more mass our boat 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.
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.
At the end of 20 miles, when we stop paddling, all the kinetic energy will be converted to heat as the boat coasts to a stop. Kinetic energy is a red herring!
The Carry Factor Strict mathematical modelling doesn’t negate the Carry Factor. Oversimplification does. When we lift a weight potential energy is stored in the weight. When we put it down that energy is released. Unfortunantly our muscles are not 100% efficient and they do not regenerate energy when operated in reverse. We burn calories both when we lift a mass and when we put it back down. This is the principle that makes Step Aerobics work. For minimum use of energy we could lift our boats and carry them to the water and drop them in from the carrying position.
Is it easier to carry an extra 150 pound in a backpack or in a kayak? Of course it’s easier in a kayak. The only effect that the extra weight has is a slight increase in wetted area and drag. The total drag on a kayak is in the neighborhood of 3-5 pounds depending on speed and wetted area. And this is the same force our arms apply to the paddle to overcome drag. Add 150 pounds to a backpack and almost no one will be able to stand wearing the backpack. This is becuse our muscles have to directly lift that 150 pounds and 150 is a much larger number than 3-5. When we walk our center of mass drops slightly with each step. We do work to raise it back up to get ready for the next step. Weight is a significant penalty when walking, but much less of one in a kayak.
What a can of worms!!!
| |
Kayak and Canoe Design Bulletin Board is maintained by Nick Schade - Guillemot Kayaks with WebBBS 5.12.
|
Boat Design |
|
|
|
