[Cloud666]

Hello, am a 3rd year civil engineer and I had a bet with my friend who studies mechanical engineer about an issue and we need professional help.

for me it was a physics crime! The story is.

an escalator runs on a motor "on electricity" he wants to hook a shaft to the escalator and the shaft is hooked to a generator to a DC battery and an inverter to change DC to AC, he claims that he can re-run the escalator and stop using the motor, the argument was that the shaft can turn the 50 rpm to 500 rpm and produce more power than he actually has drained from the motor!!

I kept telling him that the Power=Torque.RPM and if u change the rpm it will effect the torque because the Power is constant.

could you give me a logical answer I can reach him with the argument got to a point that he shouted that I know nothing about mechanical engineer and I can't argue with him!

[ebaines]

Your friend is wrong - what he is proposing is a perpetual motion machine, so you know that it can't possibly work. The reason it can't work is not that "power is constant" (whatever you were trying to convey), but rather that the power required to turn the shaft of the generator is greater than the electrical power that it produces - energy can't be created out of nothing, and all mechanisms have some amount of loss inherent in them. Then that electrical power drives the escalator motor, but the mechanical power available from the shaft of the motor to move the escalator is less than the electrical power that goes into it. Now add the friction of the escalator mechanism, which robs more power, and the energy required whenever a person steps on the escalator to go up to the next floor. You can see that the energy lost in the system makes it impossible to be self-sustaining; some amount of outside energy is required to make this work. When your friend takes thermodynamics as part of his ME curriculum he'll get a better idea of why his suggestion is impossible.

By the way - if an esacaltor or elevator could be made to operate for free this way, don't you think the escalator manufacturers would be all over this? Imagine how they could increase sales and profits if they could market an energy-free machine.

[Cloud666]

Exactly am thinking the same thing and I've told him everything you said but I didn't mention the friction of the escalator mechanism, he is in his fifth year and I used to think he is smart not after this theory of him , thanks man :) now am sure about my argument.