Physics Car

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Keand64
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Physics Car

Postby Keand64 » Thu Dec 16, 2010 1:43 am UTC

For a physics project, I have to make a car that runs ten meters or so. I've a couple ideas already, I just figured, the people here are smart, they prolly have some ideas themselves. So, if you have any suggestions or ideas for a car, then go ahead and post'em.

There were some limitations on the materials and whatnot, like no batteries/electricity, etc, only using string, some wood, and a couple other misc. things, I'm not gonna give the full list, because I'm not looking for someone to be all like "this is what to do: instructions", I'm really just wondering what sort of general ideas you have..
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Re: Physics Car

Postby OBrien » Thu Dec 16, 2010 1:49 am UTC

I'm assuming this is amatchbox-type model car you'll be using, not a real one? And are you allowed to have "launch pads", or does it have to be entirely contained in the car? If so you could try to make a rudimentary crossbow. If not, how about the good ol' wind-up-elastic technique?
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Re: Physics Car

Postby PhoenixEnigma » Thu Dec 16, 2010 1:52 am UTC

Wrap a string around the axle, up over a pulley, and tie it off to something heavy. Heavy object descends, string gets pulled, spins the axle as it unwinds, and car goes forward.
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Re: Physics Car

Postby Kang » Thu Dec 16, 2010 9:59 am UTC

No electricity? MacGyver and I are offended! Well, instead burn the wood to make it steam-powered then.

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Re: Physics Car

Postby Hawknc » Thu Dec 16, 2010 10:29 am UTC

Keand64 wrote:For a physics project, I have to make a car that runs ten meters or so. I've a couple ideas already, I just figured, the people here are smart, they prolly have some ideas themselves. So, if you have any suggestions or ideas for a car, then go ahead and post'em.

There were some limitations on the materials and whatnot, like no batteries/electricity, etc, only using string, some wood, and a couple other misc. things, I'm not gonna give the full list, because I'm not looking for someone to be all like "this is what to do: instructions", I'm really just wondering what sort of general ideas you have..

Most people who have done a physics/engineering course will probably have designed something like this at some point, so I'm sure you won't be short of ideas, but it really depends on what other constraints you've been given. Mousetrap cars are pretty popular - the springs inside them store and release energy efficiently, and you can get a wide variety of sizes to suit your needs. Otherwise you could use falling weights, compressed air, some kind of elastic launcher (like the crossbow mentioned earlier)...it's up to you and the rules, really.

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Re: Physics Car

Postby OBrien » Thu Dec 16, 2010 11:43 am UTC

Please note that I'm a mathemagician, not an engineer, and I only mentioned crossbows because they are awesome. And the wind-up-elastic thing because it's tried and tested and is one of the few things I know that actually work.
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Keand64
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Re: Physics Car

Postby Keand64 » Thu Dec 16, 2010 9:03 pm UTC

OBrien wrote:are you allowed to have "launch pads", or does it have to be entirely contained in the car?

Has to be contained completely in the car, unfortunately. So no crossbows.

Phoenix Enigma wrote:Wrap a string around the axle, up over a pulley, and tie it off to something heavy. Heavy object descends, string gets pulled, spins the axle as it unwinds, and car goes forward.

I like this idea a lot, I think I might do something like it, prolly do something to prevent it from falling to fast though, and running out after the first couple feet.

Kang wrote:No electricity? MacGyver and I are offended! Well, instead burn the wood to make it steam-powered then.

Haha, I'm not sure that's allowed either, though.

Hawknc wrote:Most people who have done a physics/engineering course will probably have designed something like this at some point, so I'm sure you won't be short of ideas, but it really depends on what other constraints you've been given. Mousetrap cars are pretty popular - the springs inside them store and release energy efficiently, and you can get a wide variety of sizes to suit your needs. Otherwise you could use falling weights, compressed air, some kind of elastic launcher (like the crossbow mentioned earlier)...it's up to you and the rules, really.

Yeah, I do have a couple ideas, I really just wanted to see if anyone here had some ideas they wanted to share so I could steal utilize them. Might post the finished car if I end up liking it.

OBrien wrote:Please note that I'm a mathemagician, not an engineer, and I only mentioned crossbows because they are awesome.

They are.
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Re: Physics Car

Postby Thesh » Thu Dec 16, 2010 9:16 pm UTC

I'm assuming you can't tie the string to the front and then pull it?

I vote for the pulley/weight idea. For the speed at which the weight falls, you should be able to arrange the pulleys so that the weight falls at the rate you want.
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Re: Physics Car

Postby ArgonV » Thu Dec 16, 2010 9:20 pm UTC

Use a fire extinguisher? Coke and mentos? Maybe turn a can of lighter fluid into a jet engine?

Yes, I've done all of these things

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Re: Physics Car

Postby You, sir, name? » Thu Dec 16, 2010 9:54 pm UTC

You could go with gravity and conservation of momentum. Put something (water?) in a long J-shaped pipe with a narrow end, and then as it falls down the tube and gets redirected at the end of it, there will be a forward-acting force on the pipe.
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Re: Physics Car

Postby OBrien » Fri Dec 17, 2010 12:46 am UTC

ArgonV wrote:Use a fire extinguisher? Coke and mentos? Maybe turn a can of lighter fluid into a jet engine?

Yes, I've done all of these things

Actually, along the Coke-and-Mentos idea (ie:something in a bottle), maybe attach a bottle rocket to your car? Pretty sure it'll go a hell of a lot more than 10 meters, especially if you use a 2 litre bottle (although I'm not sure whether or not the bike-pump part will be allowed under your rules...).
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Re: Physics Car

Postby EmptySet » Fri Dec 17, 2010 1:07 am UTC

If it's a smallish car and you don't have elastic, you can also use twisted rope/string to drive the wheels.

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Re: Physics Car

Postby Aetius » Fri Dec 17, 2010 1:16 am UTC

10m? Just throw it!

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Re: Physics Car

Postby EdgarJPublius » Fri Dec 17, 2010 5:27 am UTC

ramp + low rolling resistance = distance!

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Re: Physics Car

Postby Sourire » Fri Dec 17, 2010 3:46 pm UTC

You, sir, name? wrote:You could go with gravity and conservation of momentum. Put something (water?) in a long J-shaped pipe with a narrow end, and then as it falls down the tube and gets redirected at the end of it, there will be a forward-acting force on the pipe.

I'm really intrigued by this idea. Can you provide a link to this in practice, or give a diagram? I'm not sure I understand what you mean from this chunk of text, but it sounds awesome.
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Re: Physics Car

Postby Keand64 » Fri Dec 17, 2010 8:37 pm UTC

some people wrote:I'm assuming you can't tie the string to the front and then pull it?

Use a fire extinguisher? Coke and mentos? Maybe turn a can of lighter fluid into a jet engine?

Put something (water?) in a long J-shaped pipe with a narrow end, and then as it falls down the tube and gets redirected at the end of it, there will be a forward-acting force on the pipe.

10m? Just throw it!

ramp + low rolling resistance = distance!


unfortunately, none of these are allowed. I think I'm gonna do a combination between a falling weight, and a mousetrap car, where the weight triggers the mousetrap when it falls all the way down. When I finish it, I'll post the final version and the full materials list, so y'all can see the limits I was limited to.
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Re: Physics Car

Postby EmptySet » Sat Dec 18, 2010 12:28 am UTC

Sourire wrote:
You, sir, name? wrote:You could go with gravity and conservation of momentum. Put something (water?) in a long J-shaped pipe with a narrow end, and then as it falls down the tube and gets redirected at the end of it, there will be a forward-acting force on the pipe.

I'm really intrigued by this idea. Can you provide a link to this in practice, or give a diagram? I'm not sure I understand what you mean from this chunk of text, but it sounds awesome.


Basically, you stick a bottle on the car with a nozzle on the bottom, pointing backwards. Gravity forces the water downwards, pushing it through the nozzle and thus causing it to shoot out the back of the cart. Now, there is no net force on the whole cart/water system - gravity is balanced by the normal force of the floor pushing up - so the combined momentum of the cart and water must remain the same. Thus, when the water sprays backwards, the rest of the cart is pushed forwards to balance it. "Recoil", if you will. The principle is basically the same as a rocket or fan, but rather than being driven by a motor or stuff exploding it is driven by gravity. Oh, and the pipe should have a narrow end because this causes the water to spray out with high velocity rather than just dribbling out, which gets you more thrust. Also there's some friction and displaced air and whatnot which I am ignoring for the sake of simplicity.
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Re: Physics Car

Postby You, sir, name? » Sat Dec 18, 2010 12:41 am UTC

Yeah, that's it. I think it rather elegantly demonstrates several fundamental principles of mechanics: All three of Newton's principles, as well as conservation of energy (where in this case, potential energy is visually indicated by height of water in the tank). And the fundamentally irreversible nature of the process also hints of the second law of thermodynamics.
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Re: Physics Car

Postby General_Norris » Sat Dec 18, 2010 9:40 am UTC

Put your dog in front of the car, of course =D

You can try a Stirling engine, they are easy to make and look cool. It's like having a steam engine without steam!

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Re: Physics Car

Postby silent man » Sun Dec 19, 2010 7:23 pm UTC

- Container with nozzle-like opening + liquid nitrogen (or dry ice for people who are on a budget/fond of their fingers) + heat source = reaction drive

- Mount a crossbow-(or ballista-)mechanism on your car that pushes against a solid object (like a wall or your foot). If that is not allowed, it might be possible to make a "foot" that provides a solid point to push off of and then lifts up, like someone riding a skateboard.

- A spiral spring from an old alarm clock or such can deliver quite a bit of torque and might even last the whole way. If not, install a free-wheeling mechanism.

- Use a series of "legs" powered by coil springs and mounted on an axis to repeatedly kick against the ground at an angle. (Awful sketch spoilered because I suck at using Paint)
Spoiler:
crazydrive.JPG
I call it the Marty-McFlywheel...

- If compressed gas sources (like those compressed-air-cans for cleaning computers) you could drive a small turbine.

ArgonV wrote:Use a fire extinguisher? Coke and mentos? Maybe turn a can of lighter fluid into a jet engine?

Yes, I've done all of these things
Either you meant to say rocket engine, or you need to post some pics, 'cause that would be insanely awesome.

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Re: Physics Car

Postby ArgonV » Sun Dec 19, 2010 7:40 pm UTC

Alas, I meant to say rocket engine :P

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Re: Physics Car

Postby Plasma Man » Mon Dec 20, 2010 12:10 pm UTC

This is probably too complicated / dangerous, but if you could build a long, airtight piston and cylinder, you could attach a string to the piston and heat the cylinder. Once the cylinder is as hot as you think you can cope with, put the piston in. Then, as the cylinder cools, the air inside should decrease in volume, drawing down the piston and pulling on the string, which could then be used to drive an axle or wheel. It would be slow, but it's not the most obvious approach. Of course, if you wanted to speed it up, you could put You,sir,name?'s / EmptySet's funnel round the cylinder and use iced water as the liquid.
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Re: Physics Car

Postby cooky173 » Tue Dec 21, 2010 12:15 pm UTC

Can you make a pulse jet engine?

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Re: Physics Car

Postby osiris32 » Tue Dec 21, 2010 9:02 pm UTC

I've seen a mouse trap car that not only went well past the 10 meter mark, but did so at a rapid pace. Just tie a string onto the bar of the trap, and wrap that around the axle. Gear that axle down to the actual drive axle (I've seen as much as 20:1) and you have a very strong power source. I'd also recommend very good bearings on your axles, low friction tires, and for the conservation of momentum, higher weights.

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Re: Physics Car

Postby Jacque » Tue Dec 21, 2010 10:52 pm UTC

I remember doing a mousetrap car back in high school physics.

We had to have the car travel 10 meters, stop w/o going over the 10 meter mark. Then utilizing a second mouse trap, launch a piece of cheese up 5 stairs on a staircase.

My solution was a stupid-simple car of built out of k'nex I had from my childhood in somewhat the shape of a dragster; long, little wheels in the front, big wheels in the back. Had one mousetrap on the front with a dowel taped to the metal snapping part, and a length of string tied to the dowel and wound around the back axel.

A second length of string was tied to the back axel and the other end created a small noose like loop around the trigger mechanism of the second mouse trap. This second length of string was exactly long enough that when it wound around the axel (as the other length of string was unwound), it would cause the axel to lock-up at exactly 10 meters which would cause the noose to tighten and launch the payload at the same instant.


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