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From: Niels Blaauw <niels.blaauw_at_wanadoo.nl> subject: Re: [Paddlewise] Energy, force and work Date: Thu, 12 Dec 2002 18:06:00 -0800 · This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:33:32 PDT

According to Matt it takes a constant supply of energy to keep a boat
lifted. According to Peter is does not. Actually, they are both right.
It depends on the circumstances. The main problem is their choice of
examples: Airplains and musclepower are clearly too complicated to bring
their views together.

Let's consider a new example: A helicopter, flying, but not moving.
According to Peter, the helicopter does not use energy to stay in its
place. Correct! However, when the engine stops, the helicopter will fall
like a brick. Why? Because the helicopter may not be using energy to
stay in place, it DOES use energy to move air particles downward. The
force used to accelerate these particles keeps the helicopter in place. 

Consider an escalator, going downwards. You walk in the opposite
direction, upwards, at a speed that cancels out the speed of the
escalator. You stay in the same place, while constantly walking upwards.
Using energy? Once more: Yes and no. While there is no energy needed to
stay in place, there is constant energy needed to keep pushing down the
steps of the escalator, and if you don't, you start going down.

The same principle applies to a planing hull: To keep the hull on the
surface, the hull has to move water particles down. It uses energy to do
this.

Now, let's clear up the confusion between Matt and Peter. First the
airplane. Once more, the plane has to move air particles down to stay at
the same height. The amount of energy used might be small compared to
the amount of energy used to keep the plane at a constant speed of 500
miles/hour, so when you calculate the engine power of an airplane, the
energy used to keep it in the air might be insignificant, but still, it
is there.

In the example of Matt, lifting a kayak from a pair of sawhorses, you
don't need energy to keep the kayak in place. However, muscles
delivering a force are not stationary. Muscle cells can supply force for
only a very short time, so to keep the force up, they are taking over
from each other. Constantly cells are contracting and releasing. In a
way, every cell is in constant vibration, loosing energy on friction.
Energy used? Yes. 

In all examples, energy is delivered. This energy does not result in
motion or translation, at least not for the object we're talking about.
We only move secondary objects: Air particles, water particles,
escalator steps or muscle cells. All these secondary objects will pass
that energy along, by supplying a breeze, a vibration, a sound, or just
heat. That's where all energy ends, in heat.

If you perform any action that raises your heartbeat and breathing rate,
warms you up, and leaves you tired and hungry, be sure you are using
energy.

By the way: The laws of nature explained here are commonly known as
"science". Now, contrary to what a lot of people think, the goal of
science is not to supply the ultimate truth. Science provides models,
that make it possible to predict events with sufficient accuracy. For
example: To predict the way balls move on a pooltable, you can use the
laws defined by Newton. They are pretty accurate on objects that don't
move too fast. However, if you play the game so fast that the balls
approach the speed of light, you need more laws: Time and mass are not
constants anymore, as explained by Einstein. Does that mean that Newton
was incorrect? Yes, but who cares: For any situation on earth, his laws
are accurate. Who cares if they supply an ultimate truth? 
Another neat example is the behavior of light. Some of its behavior is
explained by a model based on particles, another part is explained as
waves. So what is it? Waves or particles? It's neither particles nor
waves. Light is light. Particles and waves are just models, used to give
an easier understanding of its behavior.

In the end: There is only one ultimate truth. It is fully explained in
"the Hitchhikers Guide to the Galaxy", as "the Ultimate Answer to Life,
the Universe and Everything". The answer is 42.

The same applies, in my opinion, to the writings of Matt: He is not
supplying an ultimate truth, but he is supplying accurate models to
predict the behavior of a kayak. Thanks Matt!

Niels.
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