# Homework Help: Science Fair - Why perpertual motion won't work.

1. Feb 16, 2012

Hi,

I am thinking of doing my next science fair project on perpetual motion, showing what prevents it from working. I am thinking of getting a 1-2 foot diameter plastic tube, maybe 3 feet long, mounting one electric fan about 1 foot inside one end, and another unpowered fan one foot inside the tube from the other end. My idea would be to connect a generator to the unpowered fan so that the powered fan would blow, spin the unpowered fan, and generated some power. I would discuss the reasons that the powered fan, spinning the unpowered fan, can't generated more power than it takes to run the system.

I won't be able to start my project until my birthday, when I can buy the materials, but I have some questions. I don't want to do this unless I can show that the power generated is somewhat close to the input power. I mean, it won't make sense if the output is 70% of the input. Can anyone tell me if this idea could end up outputting at least 90% of the input power, without having to buy some uber expensive fans and generators?

Thankies,

JM

Last edited: Feb 16, 2012
2. Feb 16, 2012

### cepheid

Staff Emeritus
Welcome to PF,

Okay. So basically you want to demonstrate the conservation of energy. The energy you get out can't be greater than the energy you put in.

What won't make sense?

3. Feb 16, 2012

Not that it would not make sense, just that I would rather have the project result in over 90%. I appreciate the reply but you did not answer my question.

JM

4. Feb 16, 2012

### cepheid

Staff Emeritus
Yeah, I get that you want a result of over 90% efficiency, but I'm just trying to understand your motivation for that. Why does it matter? How is it important to illustrating your point?

Off the top of my head, I don't know the answer to your question. However, if I had to guess, I'd think that achieving 90% efficiency would be unlikely with such a setup.