Here's a question

[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

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]quote:Originally posted by don c.

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).