# Way over my little high school headhelp!

starspin213
ok: so i got this project for physics, and we have permission to ask ANNYYYOONNE we want for ideas/help...and well, please help! we have to build a car that can travel a distance of 8-12 meters. it cannot be propelled by human, animal, or commerical propulsion. it must stop automatically. it cannot be tethered or controlled by remote control. it must fit in a 1 meter cube.
purpose: on the day of testing, the teacher will pick 1 of 400 distances between 8 and 12 meters. we have 5 minutes to then configure our car and make it "go" and reach the distance. the car with the fastest time and distance closest to the mark wins.

help. we dont know where to start. weve thought. weve wracked our brains and all we have is...well..wracked brains. pleaasse help!

Originally posted by starspin213
...we dont know where to start. weve thought. weve wracked our brains and all we have is...well..wracked brains. pleaasse help!
Just how many of you are there?

Let me make sure I understand; You need a quick burst of movement followed by some hard braking?

Hmm, the braking is what makes this interesting to me. I have a couple of ideas already but need to ruminate on them a little.

Oh, and welcome to PF, starspin213 !!

starspin213

Just how many of you are there?
theres 3 of us... and the only idea we have so far is a rubber band car...lol.

Let me make sure I understand; You need a quick burst of movement followed by some hard braking?
correct...but we dont know what distance we have to get the thing to stop at until the day the project is due..crazyness.

Oh, and welcome to PF, starspin213 !!
thank ye!

I'd likely ditch the rubberband idea in favor of compressed air. To be quick off the line and brake more rapidly, a lightweight vehicle would be a good idea. Compressed air inside a large, plastic soda bottle(s) would make for a powerful lightweight motor too
Perhaps with a consistant starting pressure (charged with a bicycle pump, for example) a simple timer of some sort could be made to divert the airflow from powering the vehicle to locking the brakes after a given number of seconds determined from practice to cover various distances.

starspin213
wow...that sounds like a spiffy idea. but would a group of 3 physics challanged girls be able to do that (i dont want to start and fail and waste time and money..).....?

Well the beauty of a cardboard box on wheels being propelled by an empty Coke bottle charged with air is that your overhead is not very high.

On second thought, maybe that rubberband idea of yours wasn't so bad after all. Check this out;

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Homework Helper
Couple'a questions:

Does the car have to stay within the cubic meter during the race?
How precise must the motion be?
Does the time spent setting it up count?
What is 'commercial' propulsion?
Where will the race be taking place?

starspin213
Originally posted by NateTG
Couple'a questions:

Does the car have to stay within the cubic meter during the race?

no, it just has to fit in the box initially

How precise must the motion be?
the more precise, the better. the closer we get to the objective mark, the better score/grade.

Does the time spent setting it up count?
no, the time that counts in the score/grade is how long it takes the car to start, travel, and stop
What is 'commercial' propulsion?
any kind of engine, something requiring a push or pull start

Where will the race be taking place?
the tiled hallway of my school

thanks

In order to stop fast, you'll want a braking mechanism. You don't want to rely on the car coming to a stop. Whatever drive mechanism you choose, install a couple of spools.

On one spool is a length of cord corresponding to the distance you want to travel. Note, the length will not necessarily equal that distance, there will be some factor determined by the ratio of the wheel size to the spool size.

The other spool is linked to the car's drive axle, and takes up the cord. When the cord is completely transferred, the axle can no longer turn.

Time line:

-Learn distance to be travelled.
-cut a piece of breaking cord to correspond to that distance
-tie the cord to the supply spool and axle spool, and wind it around the supply spool.
-wind rubber band as tight as possible without breaking it
-let car go.
-as car travels, cord is taken up by axle spool, rubber band unwinds
-long before rubber band finishes unwinding, car reaches destination, supply spool runs out.
-car jerks to a sudden stop.

This design has the advantage of using the rubber band most efficiently.

Njorl

Homework Helper
If you have them, Lego Mindstorms would be an excellent choice for a timing-based approach. Probably runs foul of the no engine rule though :(.

A second option is to get a car with adjustable drag, and an inclined plane to launch it with, or a car with fixed drag and markings on the inclined plane.

I suppose that putting a suction cup down and using rope would be considered cheating (tethered)? Even if the rope is used to trigger breaking?

A slick solution would be to use a bunch of hinged sections in an accordion fold that allow the car to stretch to the right length when you release it.

If you're feeling adventerous, you could build an air-cannon launched projectile car. It's probably easier to controll distance balistically if you're going for speed. I'm not sure what you've got avialable, but it's important to be able to controll the pressure well.

A relatively simple method would be to use a wound rope and dropping weight to drive the car. It would provide you with an automatic control on the range.

Do you have any idea about what materials you'll use?

starspin213
ok..first, thanks for all the ideas yall, you have no idea how much my team and i appreciate the help

Originally posted by NateTG
A relatively simple method would be to use a wound rope and dropping weight to drive the car. It would provide you with an automatic control on the range.
say what?

Do you have any idea about what materials you'll use?
no, but we've thought of wood, like one of those little racer cars you make in like 3rd grade...but who knows

Homework Helper
Pulley

If you attach the end of a string to the axle, wind it up, and then put it over a pulley, then a weight on the other side of the pulley would unwind the string, and move the car forward.

If the string length is set up well, then the string will run out while the weight is still dangling.

Once the string has run out, the axle will be winding up the string, pulling the weight back up, and slowing down the car.

If you put a ratchet on the axle, then you can prevent it from rolling backwards at the end of the run.

This arrangement will essentially provide you with a constant acceleration, and then a constant deceleration to a stop.

You can control the range of the car with the amount of winding you start out with.

starspin213
ohhh...ok. i get it now! thanks

barcat
Wouldn’t it be nice if they could rig a “Clock-Spring” type drive that unrolled a string or rope from around a turning axel? The braking would be as simple as measuring the desired distance of the rope and applying a stop at that point. The rope would be pulled through a hole and would have knot in it at the distance calculated.

starspin213
so the other end of the string would just wind around a spool or some such thing...and at you would just time a not at the distance you want it to stop at?

barcat
Oh Yes! We made something like this in HS. We used a spring starter assembly that we got from one of those old wind up lawn mowers. I'm sure that one of those small engin junk yards would still have one. We assembled the spring assembly on the front axel so that the tention would wind the rope on the frot axel as it was unrolled off of the rear axel. All we had to do was to tie a knot/loop in the rope. Then push the cart backwards the distance we wanted it to travel. This would wind the spring tight enough for just enough acceleration. Once the knot hit the hole in the board that was between the axels, it was just like slamming on the brakes.

starspin213
Originally posted by barcat
Oh Yes! We made something like this in HS. We used a spring starter assembly that we got from one of those old wind up lawn mowers. I'm sure that one of those small engin junk yards would still have one. We assembled the spring assembly on the front axel so that the tention would wind the rope on the frot axel as it was unrolled off of the rear axel. All we had to do was to tie a knot/loop in the rope. Then push the cart backwards the distance we wanted it to travel. This would wind the spring tight enough for just enough acceleration. Once the knot hit the hole in the board that was between the axels, it was just like slamming on the brakes.

sweet! im really liking the string w/ knot thing....oh btw, is there a way to use the spring thing without moving the car out of our 1 meter box?

barcat
I would'nt see you not. Just lift the end of the box that has the spring and "re-wind" the wheel. Or is that a No-No!

Flywheel

It is quite common in Europe to "charge" delivery trucks by spinning half-ton flywheels to over 100 revolutions per second, and letting this built-up energy carry the truck to its destination.

For a project where you only need to go 12 meters, assuming your car has a mass of, say 5 kg, you could spin a 2 kg flywheel in the neighborhood of 10 revolutions per second. You can do this with an electric motor at the beginning of the run.

For the brakes, using any car, I would suggest the following mechanism, though I am no expert and this may not work: Tie to the flywheel a length of loose wire, and attach the end of this wire to the frame of the car itself. Be sure that you are using strong wire and not rope or it will simply break (not brake!). While the car is moving along, the length of wire will loop itself around the flywheel, and eventually it won't be a loose wire anymore. At that time, the car will jerk backwards and (hopefully) stop.

EDIT: After more carefully reviewing previous posts, perhaps it would be better to punch a hole in the platform, thread the wire through, and tie a knot on the other side. Either way would work.

When you are building the car, set up the wire to brake at 12 meters. When you have your five minutes to set up the car, depending on how far you have to go, you can loop the wire around the flywheel once or twice to begin with. Perhaps it would be a good idea to have multiple attachment points for the wire on the flywheel. After you are done with that, you would get out an electric motor and spin the wheel as fast as you can.

Then, the car will take off, zoom along the hallway while slowing, and finally jerk backwards and brake.

I also want to emphasize that this would not be a terribly complicated thing to build. I mean, sure, a cardboard box with a Coke bottle on top is easier, but you don't have to be Wernher von Braun to build this either. You would basically have a wooden platform as the foundation. You would mount the flywheel with its length of rope on the rear side of the platform. You would take the other end of the length of wire, and attach it (with a strong adhesive, such as superglue or duct tape) to the front side of the platform. Then you would put your wheels and axle on the rear side and the front side, and finally stretch a rubber band between the axle of the flywheel and the axle of the rear wheels.

Just my 2 cents.

--Psi

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starspin213
hey yall... me again... well, i ended up making a little formula one car, basically... and its gonna start with a ramp (hopefully) and stop with a string.... and i had a paper that gave a formula, basically, of how to figure out the length of the string. something with the circumfrence of the axel divided by distance and then something else... and i lost it...

does anyone know how to figure it out? any help would be so much appreciated..

<- me in physics class (i love it, but it confuses me madly..)