Here's a question

[J10]

How could you drive a generator with a tension on a rope?

For instance if you had a rope with a steady pull applied to it, how could you have a generator or flywheel fixed to one spot to utilize that pull to generate electricity? Let's say that having the rope move back and forth would be possible but not the best solution, how would that be done?

Ya that's right, I'm back.

[don c.]

quote:Originally posted by J10

How could you drive a generator with a tension on a rope?

For instance if you had a rope with a steady pull applied to it, how could you have a generator or flywheel fixed to one spot to utilize that pull to generate electricity? Let's say that having the rope move back and forth would be possible but not the best solution, how would that be done?

Ya that's right, I'm back.


Quick answer = you couldn't. You can translate the rope tension to a pressure transducer to create a voltage, but you haven't performed work. Lifting a barbell requires work; and holding it over your head does require continued energy input, but you haven't performed work, according to Newton. "Generating electricity" means creating electromagnetic force, EMF (Volts) times current flow (Amps) to produce power (Watts).

[Casey]

I'm sure no physics major. And before reading don's answer I was trying to visualize that. But just by experience I can not imagine what you are suggesting. There would have to be motion involved to turn the generator and the rope tension would be static.

Edit: You would get 1/2 turn when the pull was first applied to the opposite side of a wheel, and that might generate electricity for that half turn, but it would stop when the rope was on the side of the wheel closest to the attachment point of the rope. 1/2 cycle.

[hodgepoj]

I want to reply to both Don and to Casey.

Both of you are partially correct in that generating electricity requires a conversion of energy from some other form (mechanical, chemical, thermal, radiant, nuclear, acoustical, etc.) into electrical form. If mechanical energy is being converted into electrical energy, then work is being done. Work is a mechanical concept. It's what happens when a force is applied over a distance.

Don, lifting a barbell indeed requires work and although holding it over your head makes your muscles tired it does NOT require work (or continuous energy input, as you call it). You can apply a force all day long but if the object you are applying the force to does not move in the direction of the force, then you have done no work on it and its energy state has not changed.

Historically, Newton's laws of motion do not address the concept of work or energy. These ideas came along 100 to 200 years after Newton.

Casey, I AM a physics major. There would indeed have to be motion in order to turn a generator (rotation is, after all, a form of motion). But tension in a rope need not be static. The rope in a block and tackle certainly has tension in it when you lift a heavy weight, but it's not static when the pulleys are rotating.

J10 did not provide enough conditions in his question to enable a complete answer. He said the rope has a steady pull, but he didn't say it was stationary. His flywheel could be fixed to one spot but could still generate electricity if it were rotating about a fixed axis. In that situation, a continuous rope or belt wound under tension around two wheels could indeed generate electricity if the wheels were rotating.

If he demands that the rope be stationary as well as being under tension, then both of you are correct.


Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210

[don c.]

quote:Originally posted by hodgepoj
Don, lifting a barbell indeed requires work and although holding it over your head makes your muscles tired it does NOT require work (or continuous energy input, as you call it).

hodgepoj, I didn't call "continuous energy input" work:" Lifting a barbell requires work; and holding it over your head does require continued energy input, but you haven't performed work"". Continuous energy input in the case of holding a barbell over your head, or pressing against a wall, is "force", not "work".

quote:His flywheel could be fixed to one spot but could still generate electricity if it were rotating about a fixed axis. In that situation, a continuous rope or belt wound under tension around two wheels could indeed generate electricity if the wheels were rotating.

This sounds like a description of an automotive alternator setup, with the v-belt being replaced by a rope. Nobody, however, would seriously call the belt itself the electrical generator.

[Seeker]

This topic sounds a bit similar to 'perpetual motion machines'.

I realize that a perpetual motion machine has never been shown to work, and probably never will be...

But sometimes it's fun to try to think one up just the same.

Here's what mine would look like...

You have a rope hanging over a pulley, with boxes tied to both ends of the rope, so one box is hanging to the left of the pulley, and the other to the right of it.
Then there are balls found in the boxes . The boxes could pick up or drop balls, either when the rope pulls them to a higher height, or to a lower height (the floor).

When a box touches the floor, the floor would knock against a stick which is keeping the balls in the box.So when the stick contacts the floor, it releases ball(s) into a catch at the floor level. There would be a sloped board inside the box which a ball would run down, onto the floor.

The boxes would need to 'know' when it was advantageous to pick up or drop balls at a position above, or when to do so at the floor level...so that the system is always 'out of balance'( with respect to the weight of the right box vs the weight of the left box).

Like I said, it's fun to try to imagine a perpertual motion machine sometimes. But getting one to work...now that's another story !

Seeker

[J10]

Great answers, I'm not looking at perpetual motion so much as just pondering ways to harness existing forces though. What I am looking at is harnessing wind power, nothing more than a mental exercise really. I watch people with rather smallish $50 kites being literally dragged across the beach sand here and wonder if there are better ways to harness that energy than expensive windmills. An extended sail can catch much more wind energy than a windwill blade as well. I've seen the windmill farms that surround the SF Bay Area many times and always notice the rather smallish effective surface area in all of those acres of windmills, besides the obvious observation that they are highly expensive and seem to be down more than they are running, it's rare to see half of them spinning at any given time. I wonder how many of those windmills it would take to power an electric boat, as opposed to a few simple sheets of cloth on a sailboat. That's where I'm looking.

There must be some way of having a kite fold back upon itself to drive a generator, like a birds wing does on the upstroke to avoid giving up the energy it created on the downstroke.

Ya that's right, I'm back.

[hodgepoj]

quote:Originally posted by don c.

hodgepoj, I didn't call "continuous energy input" work:" Lifting a barbell requires work; and holding it over your head does require continued energy input, but you haven't performed work"". Continuous energy input in the case of holding a barbell over your head, or pressing against a wall, is "force", not "work".


Au contraire, Don. Any mechanical transfer of energy, whether continuous or intermittant, is work. In fact, work is defined as "the mechanical transfer of energy." Exerting a force of any magnitude or for any duration on a stationary object does no work or transfers no energy. If the object you're exerting the force on moves in response to that force, then you are doing work and transferring energy.

Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210

[don c.]

quote:Originally posted by hodgepoj

quote:Originally posted by don c.

hodgepoj, I didn't call "continuous energy input" work:" Lifting a barbell requires work; and holding it over your head does require continued energy input, but you haven't performed work"". Continuous energy input in the case of holding a barbell over your head, or pressing against a wall, is "force", not "work".


Au contraire, Don. Any mechanical transfer of energy, whether continuous or intermittant, is work. In fact, work is defined as "the mechanical transfer of energy." Exerting a force of any magnitude or for any duration on a stationary object does no work or transfers no energy. If the object you're exerting the force on moves in response to that force, then you are doing work and transferring energy.

Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210


Either I'm not expressing myself clearly, or you are not bothering to read my posts carefully. What I tried to say, is that moving a barbell or any other weight upwards, is performing work. holding the weight motionless over your head performs no work!

http://www.chem.morrisbrown.edu/faculty/okoh/notes/psc102/topic_05/work_energy.html

[Casey]

I'll throw a little fuel on the fire.

Because you can not possibly hold that weight "motionless" over your head, you are continually performing a small amount of work to keep the weight up there.

[Eddie]

Hi J10
quote:Originally posted by J10

What I am looking at is harnessing wind power, nothing more than a mental exercise really. I watch people with rather smallish $50 kites being literally dragged across the beach sand here and wonder if there are better ways to harness that energy than expensive windmills. An extended sail can catch much more wind energy than a windwill blade as well. I've seen the windmill farms that surround the SF Bay Area many times and always notice the rather smallish effective surface area in all of those acres of windmills, besides the obvious observation that they are highly expensive and seem to be down more than they are running, it's rare to see half of them spinning at any given time. I wonder how many of those windmills it would take to power an electric boat, as opposed to a few simple sheets of cloth on a sailboat. That's where I'm looking.


Windmill blades are designed to take advantage of much higher wind speeds than boat sails and operate on the same basic principle. The relative motion of the wind across the blade generates lift which propels the blade around the hub, likewise, plane wings are pulled up and boat sails are pulled across the bay. Windmills can operate in winds that are too slow to start them which may account for why you see only half of the machines working. The boat has an advantage in that it can set the sails to work in a different fashion when running with the wind, where they more resemble a parachute. There have been boats built with hard wing-like sails and others powered by wind turbines fashioned after one of the vertical axis designs either Darrieus (lift driven) or Savonius (drag driven).

Ed

[Jnadke]

quote:Originally posted by Casey


Because you can not possibly hold that weight "motionless" over your head, you are continually performing a small amount of work to keep the weight up there.

Work, as a physics term, is rigidly defined as "the transfer of energy from one system to an object in order to move that object in the direction of the force". Work is the product of the force and the distance.

Although your body may be using energy to maintain that position, it is not performing any actual work on the barbell.

[BBBix]

Well, I am in highschool and just looked in my physics book, the above definition is correct. Work = Force * Distance. If the distance moved is 0, then the work is zero.

As for the generator, if you could just have a rope that occilates back and forth, then you could have a flywheel on a one way gear system (rotates freely on the axle in one direction, turns the axel in the other (like a bicycle)). Use the pull of the wind to turn the flywheel, spin the axle, turn the generator, and tension the spring and then by maybe being able to fold the kites wings or rotate them directly into the wind (thereby reducing the pull on the rope) the spring would be able to rotate the wheel back (indipendent of the axle (like coasting on a bike)) and wind the rope back up. Then extend the kites wings or rotate them perpendilculat to the windflow and start the cycle again.

I hope that I described that clear enough....If you need me to, I can try re-describing it in a different way.

Bill B.

[Seeker]

quote:Originally posted by Jnadke

quote:Originally posted by Casey


Because you can not possibly hold that weight "motionless" over your head, you are continually performing a small amount of work to keep the weight up there.

Work, as a physics term, is rigidly defined as "the transfer of energy from one system to an object in order to move that object in the direction of the force". Work is the product of the force and the distance.

Although your body may be using energy to maintain that position, it is not performing any actual work on the barbell.


If I'm understanding what Casey's saying, I think he's saying that since it would be impossible to hold a barbell perfectly still, there has to be some motion involved, some distance covered by your arms, so in 'real life', this situation would involve some work. But, as many posters have said, if you could hold the barbell perfectly still, then there would be no work involved.

It's funny isn't it, how the term we use in Physics for 'work', bears little resemblance to how we would use this word in common every day English conversation ? If you went up to the guy straining all his muscles to the max to hold the weight up, and said, "Hey buddy, you're not working hard at all, there.." If he could squeeze out a reply, he might say something nasty, or.. he might just drop the barbell on your foot ;)

Seeker

[J10]

BBBix, that's pretty much what I am trying to solve and how I'm looking at it, lessening the resistance somehow on the against the wind stroke of a spinning flywheel or crank for a net energy gain. I understand what you are saying too Eddie, you may be right that airfoils can work at lower windspeeds than a sail, though the windmills are stuck becuase they are broken, the wind is almost nonstop and identical side by side windmills are spinning away. It seems though that the raw surface area of even a moderate sized kite could exert a cheap and substantial pull if you can neutralize the energy on the upstroke somehow.

As to the barbells, if you can hold them still and perfectly vertical with no balancing effort, I suppose the analogy is the same as when you put them back in the rack, no work is being performed to keep them off the floor. Real people need to balance the weight to keep it in a vertical position and work will always be required to keep them up, one axis for a barbell and two axes each for dumbells assuming the arms are locked. That's why people can lift much more with two hands on a barbell than with individual dumbells, you eliminate one axis of balance.

Ya that's right, I'm back.

[hodgepoj]

quote:Originally posted by don c.

[quote]
Either I'm not expressing myself clearly, or you are not bothering to read my posts carefully. What I tried to say, is that moving a barbell or any other weight upwards, is performing work. holding the weight motionless over your head performs no work!


Don, what I am objecting to is your statement that "holding it over your head requires continued energy input." Any energy input is work. Holding a weight stationary above your head requires no work or no energy input.

Dr. Paul O. Johnson
Senior Exhibit Developer
The Science Place
Dallas, Texas 75210

[Jnadke]

quote:Originally posted by hodgepoj

Any energy input is work.


Not necessarily.

Energy entering in a system comes out of a system as either heat or work (energy). Under current technology, heat is useless, so that energy is said to be lost.

The energy from the work, will enter another system, and will eventually be converted into heat (such as moving the atoms in the air displaced by the barbell, requiring work).