Self Propelled Car

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Self Propelled Car

Postby MommyMad » Fri Sep 30, 2005 2:12 pm

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.

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Postby LewisWhaley » Fri Sep 30, 2005 3:49 pm

Hi MommyMad.

This web site may be of help to your Son:

Have your Son do some Google searches using keywords similiar to these:

"self propelled car project"

"self propelled mousetrap car"

Hope this helps.

Good Luck
Best Regards,

Lewis Whaley
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Postby Taku » Wed Oct 05, 2005 10:37 pm

I did a similar project during middle school, but it wasn't really a self propelled car; it was a self propelled plane. All I had as parts were these 2 popsicle sticks to balcance itself, a little plastic propeller, and a rubber band that ties from the end of the plane to the plastic propeller. When you twist/turn the propeller, it twists the rubber band in turn. As a result (when you let go), the rubber band unwinds quickly and flys off by itself. I think you can do the same with your car project. I think it would be helpful if the rubber band is to connect to the front wheels (anything round).

Basically, my plane project speed was determined mainly by how much I wind up the rubber band.

note: this is also in another topic above
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Postby robertreavis » Sat Oct 08, 2005 12:00 pm

Lets see now...

Magnets can move the car, but what moves the magnets?
A bottle of water, doing what? If you are thinking about ejecting the water to make a rocket, you need some way to pressurise the water.

The windup "engine" using a rubber band is an excellent solution. That gives you some place to store the energy needed to drive the car. The energy itself comes from your hand when you do the winding.


Make a solid-axel drive wheel assembly.

Attach the rubber band with a hook so that when it is finished powering the car and the car is coasting, the rubber band detaches from the axel. You do not want the rubber band to rewrap itself in the opposit direction and stop and then reverse the car.
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Re: Self Propelled Car

Postby bradleyshanrock-solberg » Mon Oct 10, 2005 9:58 am

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) 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.
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Postby aznnerd666 » Mon Oct 10, 2005 6:35 pm

Hey MommyMad

A good place to start is by thinking how to create the body of the car and where to put the axles. I don't know what grade you are in, but you could easily find out that a bigger wheel lets the car travel longer per revolution. I did a similar profect in eighth grade. I used enormous chopsticks as the body of the car, along with tubing so that I could fit in an axle. My source of power was a cut rubber band that was attached and wound around the back axle. If a lot of distance is needed, I would suggest that you use a lot of rubber band if you choose to create a rubber band car. (The more wind up, the better) You could also use two sets of rubber bands to create stability.

I hope this information has been useful.

Notice that this is also in another topic above

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Postby bradleyshanrock-solberg » Tue Oct 11, 2005 12:20 pm

Note that you can not just add more rubberbands on a self-propelled car and make it go faster.

The limit of force that can be transmitted from the axle through the wheels (and thus usefully push the car forward) is the friction point of the wheels. This unfortunately is a function of the weight over the wheels (plus the wheel material and the surface on which the car is moving).

Put more force on the tires than the friction point, and the wheels spin. This is why your car wheels spin on ice even when you use the accellerator pedal normally...on a slick surface, the normal amount of power can't be transmitted to actually push the car.

If you are going for raw speed, there is a tradeoff between maximum power on the wheel and weight. Complicating this is the fact that a rubberband releases most of its energy in the first relaxation of if you want to keep accellerating at near the limit of the wheels you need to put a gear arrangement in the car, that wraps the rubberband around larger axles as the rubber band tension gets weaker (replacing raw force with leverage).

A car designed that way will accellerate at its maximum rate until you hit the largest gear, then stop accellerating and coast until the air resistance and axle friction brings it to rest.

The advantage of a catapult or slingshot arrangement in a speed contest is that it isn't limited by the friction point You can pile on more and more propulsion force and it will hurl the car forward...and do so whether the surface is smooth or grants good traction. The problem with that approach is that the rules may require the catapult to move with the car, anchoring the catapult is tricky (so the car moves, and not the catapult) and also forces are so great that the car may go airborne or flip over if not designed carefully. If accuracy is needed, the catapult tends to be tricky to aim.

Rocket-power (or equivalent, compressed can make a vehicle driven by a balloon, if it is light enough)) also doesn't depend on the friction point, but usually has the disadvantage that the propulsive fuel and its container are heavy and also just difficult to make and control.

Actually a balloon powered car might be interesting. It would have to be heavy enough to anchor the balloon, but light enough to be propelled forward, and air resistance might be a problem in the early stages.
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