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Name: Kristi
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: N/A


Question:
How can i build a solar powered vehicle using Newton's third law?



Replies:
Several science-fiction novels have described this idea pretty well. The only one I can name right off is "The Mote in God's Eye" by Larry Niven and Jerry Pournelle. You take a huge sail, miles across, made out of flimsiest material, and give it a mirror coating. Then you take it out in space and aim the sun at it. Light carries momentum, and after you've reflected light, it carries an equal amount of momentum in the other direction. Newton's Third says somebody must now have twice the original momentum of the light that got reflected. That would be you, if you're clever enough to hold on to the light sail.

If you want to build a solar-powered car, this would not be a good strategy for several reasons, one of which is that you'd only be using the light's momentum. It would be better to absorb the light, then you'd get the energy along with the (relatively negligible) momentum.

Tim Mooney


I assume you mean a vehicle that rolls around on earth, like a toy car. Newton's 3rd law just says that if you push one way, you have to push onto something. And that something is pushing back. So if your solar powered car had wheels, that were driven with an electric motor, which in turn was run from solar cells and electricity, this would do. The wheels are pushing on the ground, the ground pushes back on the wheels, and pushes the car forward. It would all the powered by the solar energy that runs the motor, just not directly, but thru electricity.

S. Ross



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