MommyMad wrote:My son has to make a self propelled car for his physics class. He is not able to use any pre-manufactured items to do this project. The two ideas he has come up with are to use magnets to get the car to move or to use two bottles with water to make it move, but I am not sure how either of these will work. Does anyone have any brilliant ideas how to accomplish this task? It will be graded on speed and distance traveled.
I notice accuracy isn't listed as important. This opens up your options quite a bit...some of the stuff listed below is very tricky to aim accurately.
The usual propulsion means in these situations is a rubber band. If rubber bands aren't allowed, anything elastic will work, even string, but won't be nearly as strong. You'll want to build a much lighter car in such a case. Catapults can work by using the frame as the "elastic" instead of the "string", but it's a ton easier if you can just use rubberbands.
The usual approach is to wrap the rubber band around an axle and then fiddle with wheel size and axle size to get the best power ratio without causing the wheels to spin. Weight over the axle helps, and you can squeeze a lot of power out of a rubber band if you vary the axle size as the rubber band relaxes, but there is a weight tradeoff for power. The ideal car of this type is accellerating for the entire length of the rubber band at near maximum capacity for the wheels (they're just a tiny bit below "spinning". In practice, it is very difficult to machine more than one or two gears on such a car (it accelerates, downshifts, accellerates), although I've seen one design with kind of a cone-shaped axle. For distance and speed, having good bearings between the axle and the frame to reduce friction is very important.
Another approach to this is a catapult, where essentially you shoot the car out of a slingshot (again rubber band is the power source). That might score well in your test (speed and distance are only success measures) if it is allowed. It is possible to build a "bootstrap" catapult where your car drags the catapult workings behind it but again, this might be pushing the limits of what's allowed. Catapults tend to be less accurate than cars.
Magnets won't work (you are describing a perpetual motion device)....at a minimum something would have to move the magnet or ferromagnetic substance ahead of the car, and if you can do that, why bother with the magnet?
The "bottle" approach sounds like a rocket-engine idea. It can work, you can build up pressure in a bottle chemically and have it go off like a miniature cannon, and such a thing could travel quite a ways if you mounted it on a wheeled chasse. It'll behave like a catapult without the catapult - all the accelleration right at the beginning, and probably be high speed and very inaccurate. Whether it is safe is another matter. I do not know the formula to do this, but I remember my high school chemistry teacher using an old-fashioned coca-cola bottle (the kind with very long necks, much sturdier to this kind of force than most glass bottles) and a chemical soup to make a pretty impressive cannon effect. I'd say talk to your chemistry teacher and see if they have any ideas on how it might be done safely, and especially with some kind of plastic bottle (a plastic bottle doesn't tend to shatter when it ruptures, making it safer. It is however, not as sturdy, and might be more reactive so it might not work.
I guess you could also just have water pour out over a paddlewheel type arrangement but such a car would be very slow and not travel very far. It is too inefficient, and far too heavy in the early stages. Water power works cheaply normally because the water and gravity are free. On a self-contained vehicle, they aren't free, you have to carry that mass around.