Motion Generator

[rlortie]

John,

Are you sure about that? I'd have thought that holding the door open with your muscles, for instance, is burning energy and creating heat.

PS - isn't it the same as holding an object up against the force of gravity?

John C.

You are comparing bio or muscle energy with mass energy. yes you can push on something all day long, not move it and start sweating. Same goes for holding something against gravity. Push on the door and then prop it open. You are no longer using Bio energy to sustain its position.

But if I pile a stack of books such as yours on my desk, the desk does not create any heat. Boy I sure wish it did. My heating bill for this huge barn I live in is killing me.

One exception to this rule, is a helicopter, It must produce energy and heat to hover, and while hovering it is considered not to be displaying measurable work. But that is another story.

I say that if there is no relative motion, there is no resistance. and one must have resistance to create heat. To lift the book or move the door creates resistance and therefore heat. While the book is on the table it is being restrained by gravity which has gradient just as the rubber band.

Ralph

[rlortie]

Jonathan,

You slipped in on me. Good answer, wish I would have thought of putting it that way.

Ralph

[rlortie]

Energy is only used when there is motion. Statistically there is no energy spent holding an object static, even against a force, but it could be argued that there is still expansion and compression occuring at smaller levels, so some small amount of heat transfere. Still the main argument is valid.

Michael, Sorry but I think your a little subjective in your above statement. True for a mass to support a static mass no energy is spent. For a jointed bio mass such as your arm to hold something static against gravity does use energy. Put a cast on your arm and a support to ground and then you will not use energy. I would say that Bio support is using energy but no work is being accomplished.

Ralph

[ken_behrendt]

Ralph wrote:

One exception to this rule, is a helicopter, It must produce energy and heat to hover, and while hovering it is considered not to be displaying measurable work.

First you say that it must produce energy to hover and then you say it does not display measurable work while hovering.

While the height of the hovering helicopter may not change, those rotor blades on it must push a lot of air out of the way to create the lifting force that allows the fuselage that they are attached to to hover. There is much aerodynamic drag on those moving blades and a lot of energy must be constantly expended by the heliocopter's engines to overcome that drag.



There seems to be some confusion here as to what happens, energywise, when a object is held at a fixed height by another object such as a support or a human arm.

Well, imagine a 100 lb block of lead that is held three feet off of the ground by a suitably shaped wooden pillar. The block does not move.

However, when the lead block was initially placed onto the pillar, something interesting took place on a submicroscopic scale inside the atoms of the lead block and the top of the wood when they came into contact with each other. As the lead was pulled down against the wood by the force of gravity, the negatively electrically charged electrons in the atoms coming into contact began to repel each other with a force that increased as the lead block came closer to the wooden pillar.

At some point, the increasing repulsive electrostatic force applied to the lead block by the wooden pillar equalled the attractive force that gravity applied to the lead block and, at that instant, the lead block stopped moving. Thus, any gravitational potential energy lost by the lead block after it made contact with the wood was immediately "stored" as an increase in the potential energy of the repulsive electric fields existing between the contacting electrons in the lead block and the wooden pillar.

From Einsteinian relativity we know that wherever we have mass we have energy and that wherever we have energy we have mass.

In the case of the lead block and wooden pillar considered above, there will be no net change in their rest masses as they come into contact and achieve an equilibrium position. Any rest mass lost by the lead block will result in an increase in the rest masses of the electrons that repel each other at their surface of contact.


ken

[Fletcher]

My bushman's understanding - you can have a force acting in a direction but no Work is performed until it moves something. A helicopter is clearly moving the air mass thru the rotors so it is a point of relativity.

Work done is defined as Force applied over distance in the direction of that force.

Potential Energy is not added to the Work Done calculation precisely because the object of the force is not moving.

Atlas holding up the world is not technically doing any work because the world is not moving, but Atlas will sure be feeling it.

In a way it's just semantics. For work to be performed a force has to have a way to show itself by moving something, albeit spring, magnetism etc. If it's not moving it's got 100 % PE & unless we can release that object turning some of that PE into KE then we can achieve no useful work or power from that force.

[Michael]

How about this:
Resistance to flow causes heat.
Resistance alone without flow causes no heat.
Resistance alone without flow technically is not, but rather is the potential for resistance.

Yep.

[Michael]

>Michael, Sorry but I think your a little subjective in your above statement.


No. It can be shown that there are expansions and contractions occuring in the muscle mass as it tries to hold a weight static. I did cover that in my post even though I didn't adress the muscle directly.
There is work being done with the muscle and it's measurable, even though it's slight, relatively speaking.

[Wheeler]

Looking deeper into this, I think I overlooked that heat is being created in my experiment with the rubber band.
The molecular movement under stress or at rest is creating heat, even on Ralphs desk.
All things under the influence of gravity are giving off some heat as they brake down.
I think if you use classical physics, you may have a good argument that motionless rest has no heat, but we now know that quantum physics shows us that there is movement at all times.
Movement of some materials are slower than others under the influence of gravity, and this is flow and friction.

This can also be tested quickly by use of two rubber bands streched across each other and fastened to a stationary surface.
Over time one or the other rubber bands will brake.
The breakdown of the rubber band material is happening all the while you leave it attached to the stationary surface.
Heat is being generated as the stress of the material is decomposing.

Ralphs desk is slowly braking down and causing heat.

Michael had mentioned this along with John.

[rlortie]

First you say that it must produce energy to hover and then you say it does not display measurable work while hovering.

While the height of the hovering helicopter may not change, those rotor blades on it must push a lot of air out of the way to create the lifting force that allows the fuselage that they are attached to to hover. There is much aerodynamic drag on those moving blades and a lot of energy must be constantly expended by the heliocopter's engines to overcome that drag.

Careful Ken, You are getting close to the magnet on the refrigerator defying gravity but still seen as a conservative force, without doing any work. Yes the helicopter blades are creating resistance and heat and they may be considered doing work as an individual component. But that energy and work is wasted if the helicopter itself does not move. No different than a bio mass pushing on an immovable wall.

I go along with your story of sitting a lead weight on a pillar of wood, but I do not find any connection to resistance requiring relative motion to exist in your statement.

Michael, you are correct in stating that muscles defying gravity perform work by producing resistance and heat. This is what John was stating about holding the door open. Or is muscles any different from the helicopter blades creating heat to hover the craft, energy and resistance is present in both cases but no work is being done.

I would like to point out that my major contention here was not over the definition of work, but that of resistance only being present when relative motion produces it. One could also say that without some form of kinetic energy there is no resistance. Inertia must first be considered to apply motion causing resistance. This applies to any mass as well as electrons in a conductor.

There is resistance and heat in stretching a spring, once under tension there is no resistance and no more heat produced. To implode or release the tension the spring releases that energy not lost in heat.

Ralph

[rlortie]

I believe Jonathan said it best with his quote:

Resistance to flow causes heat.
Resistance alone without flow causes no heat.
Resistance alone without flow technically is not, but rather is the potential for resistance.

If he does not object I would like to refer to his potential for resistance simply as "inertia" resistance to motion, action or change.

Ralph

[Jonathan]

I do mind. I can speak of the resistance of an electrical resistor, even when there is no current, because I really mean the potential resistance, or resistance it would have if there were current. But that potential resistance is not the same as inertia.

[rlortie]

Jonathan,

Sorry, I hereby retract my explanation to your quote.

To my thinking, your potential resistance of a resister is no different than a brick laying on the floor. Inertia must be overcome to move the brick as an electrical potential is required to start the electrons through the resister.

Whether it is a electrical resistor or a resistance of mass they both can be considered potential resistance. Inertial force being that energy required to force the electrons moving or the brick sliding.

Ralph

[Jonathan]

In pure mechanics your explanation holds, but not in electronics. The resistance of a resistor is not inertial, it is frictive. The inductance of an inductor can be seen as inertial though.

[AgingYoung]

I was very curious if I thought about all this if I would be doing work. First I relaxed as much as I could and took my temperature. Then I keep the thermometer under my tongue and begin to read and think about this thread. Sure enough! My temperature rose 0.2 C. There was flow yet there was resistance and heat! I must have been working.

Gene

[Wheeler]

Good work Gene, but the real test should be done on WM2D.