A simple question - but I don't know the answer.?

I can see why it sounds like this should work, but obviously it can't.

As previous posters have mentioned, the mechanical power which is the output of the motor and input of the dynamo is torque times rotational speed (equivalant to force times distance in a liner system). The purpose of a gearbox is to trade these off - so the rotational speed is doubled, but this halves the torque, and so the power is unchanged.

I think the problem with the argument is in how a dynamo works. For what you have described to work, there would have to be no losses in the system, and so all the mechanical energy would be turned into electrical energy. In everyday life, dynamos are far from 100% efficient, and are not run at full capacity. Taking a bicycle dynamo as an example, we are used to the power produced (the brightness of the lights) varying with speed. The voltage depends on the speed, and the current is dependent on the lighting circuit. Because the dynamo is not at 100% efficiency, we never notice any effect due to torque. But because of the power drawn, there is a torque acting to slow down the dynamo.

Imagine that this is a perfect dynamo. As you increase the current drawn, this resistive torque would get larger and larger (the dynamo gets harder to turn), and so more torque would be needed to drive it. As you reach the system you describe, all the forces would balance, and the power from teh dynamo would be just enough to turn the motor, which would be just enough to turn teh dynamo etc. Of course, this kind of perpetual motion could never happen, but its a good thought experiment.

So in summary, its because the torque does affect the electrical power, but we don't see this effect in everyday life. I believe that in generators in power stations, however, this is exactly what happens - when there is too much demand, the generators would slow down and so reduce the voltage, unless other action is taken.

When you are running a dynamo, the power generated by that dynamo depend on two things: Torque and angular speed of the shaft. Due to conservation of energy at the gears if you increase angular speed the torque decreases and vice versa. So if you use gears to increase the angular speed of the shaft that will derive the dynamo, then the torque deriving the dynamo will decrease. So the power generated by the dynamo will be the same (if you neglect frictional forces, hysteresis, core loses, . . . .) But when you consider these loses, the power generated by the dynamo will be less than what is needed to run the motor. So the total effect is loss of energy to these non conservative effects. So you need some external source to counter act these loses. You can use a small generator to add some amount of energy to counter act these loses, and by calculating the amount of input energy form that small generator you can know the amount of the loses.

The fallacy is that the motor will have a greater load due to the gearing-up to run the dynamo. Consequently it will take more current, resulting in it requiring more current than the dynamo can provide.

It takes more torque to drive the gears, the power goes as torque x angular velocity. so if you increase the torque, you need to increase the power of the motor, so you need to increase the power to the dynamo.....ad infinitum

gear ratio is the problem, i.e. output needs to equal or exceed input in order to power to reticulate. Perpetual motion is the key to limitless energy... but currently just a dream.

Don't know, don't care. sorry dude. What's a dynamo?

sorry you lost me after dyna what ???

2 points. thanks for that. much appreciated