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School Project: Gravity Car Design Answered

Hey there guys... In my physics class we have a project in which we have to make a gravity-powered vehicle that will carry a given load along a track. The vehicle can only be powered by gravity (weights) and cannot use any other energy source such as elastics, springs, explosives, etc. I have started to make a K'nex vehicle but it is not very sturdy. The following is a picture of the rules and specifications. Much help is appreciated. Thanks

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9 years ago

Tell you what- to give me time to fix the wording of my objection to this principle in my head rather than posting some incoherent ramble, why don't you post a quick photo or video of your working prototype. You do have a working prototype, don't you? A simple video of it on a flat tabletop, with a spirit level or something in the background (could be just a glass of water) to demonstrate that the tabletop is level, rolling from a standstill under just its own power will be more than sufficient. Oh, and you say "This is the very first design in existence whereas the weight moves with the car yet stays in the forward quadrant of the hollow back wheel.". Oddly enough I had this same idea years ago...


9 years ago

Hate to tell you this but these are ancient and well-known perpetual motion designs. The constantly off-balance wheel is an attractive design for perpetual motion but it is impossible. A weight has to fall to do any work, simply holding a weight over the front part of a hollow wheel will not make it rotate.

Bah!! That's what you and your physics say, but it has yet to be done! It use to be considered impossible to fly

Please, please tell me that was irony and you omitted the ":P" from the end.

In case it wasn't, "It use to be considered impossible to fly" was an engineering challenge, not an unbreakable law of nature. Anyone who thought it was impossible to fly was clearly very stupid, because there were these little things with feathers on that had been doing it for a long time. A weight can only do work (note: not exert a force, the two are different) by falling. No ifs, no buts.

For going up to 1,000 grams, I don't think KNex are strong enough... I think it would be better to just use a stronger material. Also, you should really use much bigger wheels (the ones that will be powered at least). Because one turn from a large wheel would make the car go farther than one turn with a smaller wheel.

Well, puberty hits some later than others... Actually, think "rack and pinion" in a car-the pinion is a small, round gear which leaves the long, flat gear as the rack.

He said the device needs to move as one. :P guess i cant do it

After 2 hours of work I have finally finished a prototype. I made it using K'Nex. I still have to add a smooth surface for the ramp on top (Any ideas of something light, smooth, and sturdy?). The very front wheel starts in a retracted position but when as the weight goes down the ramp the wheel lowers until it hits the ground giving it more support.


For a light smooth surface I recommend Masonite. it's very commonly available and very smooth. It may be too heavy though, and if worse comes to worse I would just use a thin cardboard or plastic.

Um.. you shouldn't need one, but you could use graphite or talcum powder if you really wanted to. It would be super messy though lol.

Well, what do YOU think it looks like?

Here's an idea: Why not make your device with a built in ramp. The rules don't say anything about prohibiting ramps, so it should be legal. You would have a large wheel that is held on the ramp by a string that you cut to initiate it. The weight would be suspended on a platform from the axle of the wheel, that way it would add to the momentum of the wheel as it moved. You could even go so far as encasing everything in a box (hiding the ramp from everyone) so that when you cut a string, the wheel pushes a side of the box down and goes rolling down the finish line. See the diagram if you don't understand.


I see what you mean and it is a very good idea but I THINK the whole thing has to move as one. The people would want to see what is inside the box i guess to make sure i do not have a pinball-like-launcher inside of it :). But if my Physics teacher says it is alright I will probably go with your idea.

It's called a rack.

that's what she said, wooooohhhhhh Yeah, jtobako's right

NEW INGENIOUS IDEA it's a 15 inch in diameter wheel the weight is the gray circle. there's a trap door under the weight. The door only opens one way (see position of hinge. The weight forces the wheel to turn, the weight remains on the bottom of the wheel as momentum causes it to continue. The load that you have to carry could either be located in the center of the wheel or next to the drive weight for more torque. the trap door is there for when the wheel makes one full revolution it'll just go through the dorr, not up and around the wheel, which is impossible and it'll stop the wheel. The arc is so you can start the weight at a higher position.

gravity car.gif

Mind showing me a diagram of this?

basically a falling weight turns a worm gear (it doesn't even have to be a worm gear). That gear turns another gear (gear train bto get less torque (or more for this situation, I can't judge which you would need for this situation). The rubber band is a belt that turns the axle as if it was a pulley.

How about this-a weight pulls down on a lever (or string around a pulley), a string hooked onto the end of the lever/over the pulley goes down to a peg on the wheel axle (the rest of the string is wound around the axle). You'd have to figure out the best diameter for the axle, distance falling, amount of weight and type of tire (something with enough grip so that it doesn't just spin).

gravity car.jpeg

We cannot use any type of other energy source such as an elastic (rubberbands) :\

in that situation a rubberband would be no more of an energy source as a gear.

invent a gravity driven perpetual motion machine to drive the vehicle, win, and then trash the idea ;).

Use a giant lever, made out of alluminum or something, that pulls a string around the axel of the wheel, and use giant wheels, such as c-d's

our school does something similar, except it's with mouse traps (not fun when they close on your finger.

\ \
\ \
\ \
0 \
[] is weight
0 is pivot point for the lever.
\\\\ on the left is the bar for the lever
\\\\ on the right is the string
O is the wheel (attach the string to the axle