# COMMUNITY : FORUMS : RIDE

## School Project: Gravity Car Design

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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modernjedi6 years ago
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6 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.
6 years ago
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6 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...
6 years ago
Bah!! That's what you and your physics say, but it has yet to be done! It use to be considered impossible to fly
6 years ago
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.
Aar000n3y6 years ago
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.
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RaDiKaL (author)  DELETED_craz meanman6 years ago
We cannot use any type of other energy source such as an elastic (rubberbands) :\
6 years ago
in that situation a rubberband would be no more of an energy source as a gear.
6 years ago
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RaDiKaL (author)  DELETED_craz meanman6 years ago
Mind showing me a diagram of this?
6 years ago
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6 years ago
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6 years ago
It's called a rack.
6 years ago
that's what she said, wooooohhhhhh Yeah, jtobako's right
6 years ago
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6 years ago
Well, what do YOU think it looks like?
6 years ago
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6 years ago
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.
6 years ago
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.
Firebert0106 years ago
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.
6 years ago
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RaDiKaL (author)  DELETED_craz meanman6 years ago
He said the device needs to move as one. :P guess i cant do it
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.
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.
6 years ago
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.
Any ideas for a lube? :)
6 years ago
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.
guyfrom7up6 years ago
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.
jtobako6 years ago
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).
guyfrom7up6 years ago
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 \
O
[] 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