# Ideas for physics car design project?

1. Dec 29, 2011

### Techcom0

Hi guys. We have recently got a design 'competition' where we build a vehicle that will propel itself 1.0 m forward and then fire a projectile in attempt to hit a target that is 2.0 m away, perpendicular to the direction of travel. The vehicle will be traveling on sand, and can only move by gravitational or elastic energy, meaning no motors or batteries. We can choose our own mass.
Right now, I just want to know some possible ideas and materials to use, because I'm having trouble even starting the assignment. It would be awesome if we can discuss about some ideas, even the faintest ones, and hopefully through them, I can get this project started.
Just comments/suggestions etc will be much appreciated! I don't want any answers or anything, just to hear some of your opinions. Thank you!

2. Dec 29, 2011

### Curl

Use either an inverted pendulum (geared) or the classic rubber band method to propel a cart. Make the wheels large and thick if you're on sand, keep weight down.

3. Dec 29, 2011

### Techcom0

Thanks, yep, the sand part is quite an issue. Larger wheels from toy cars would be nice, right? Plus of course the rubber band style of propelling the car
I still think the main problem for me lies in the projectile firing. Propelling the car should be fine.

4. Dec 29, 2011

### Curl

you're not giving us all the information. Can you shoot any projectile?

get an airsoft pistol.

5. Dec 29, 2011

### Tea Jay

Can the propulsion system be used to then propel the projectile?

IE: Can the 1 m initial movement be used like a discus hurler's start?

Does the projectile need to accomplish anything other than contact, so it could be a the a fore mentioned soft air pellet, or does it have to knock something over/put a hole in a target, etc? How large is the target/how important is accuracy?

Does the initial 1 m have to be in a straight line, or can the path be curved/intersect a point at the 1 m mark, etc?

You mentioned it can only move by elastic or gravitation forces, but, not if the projectile firing was also limited to that. The spring of a spring action airsoft gun should count as elastic I would think.

Can the projectile itself have a spring, or compressed air or water, etc...and the firing mechanism simply holds it in, compressed until firing?

What is allowed as far as initiation of the progress forward or firing? Are you allowed to do one action to start it towards the 1 m point, and then do other actions to hit the target, or is it supposed to start towards the target and fire by itself?

As mentioned, on sand, you want it low, and light, with flotation type traction and good ground clearance so it doesn't bog down...and tank tread type tread, or at least 4wd, rolling ball, etc.

The sand can also complicate aiming, as you might be leaned over or tilted due to the terrain...so when you stop, your pitch/yaw might impact where your shot goes, especially if the target is small, etc.

The power and accuracy required will probably dictate the size and weight of your platform/form factors...as a stronger throw, etc, will require a more secure base....and higher degrees of accuracy, if you are NOT able to "aim" after you stop, etc...will require some sort of compensation for pitch/yaw for your firing mechanism.

Last edited: Dec 29, 2011
6. Dec 29, 2011

### Techcom0

Any projectile can be used.

Yes, as long as the propulsion system(s) are solely elastic potential/gravitational potential. It goes into a target, like a bullseye, and the bullseye area (best area) is 40cm x 20cm. Accuracy accounts for 1/3 of the final score. Launch position and mass account for 1/3 each, totalling the 1.
The initial 1m should be a straight line. If not, it would likely bounce off one of the walls on the side.
The projectile has to be propelled by elastic/gravitational too. I think an airsoft gun would work, actually...
The projectile can be anything. It can use anything. So a spring would be fine.
It HAS to fire by itself. Once launched, you cannot touch it anymore.

Thanks for all the suggestions/advice, appreciate it a lot!

7. Dec 29, 2011

### Tea Jay

So it just needs to hit the target, but not necessarily penetrate, etc...and its a 40x20 cm bullseye, do you know how high its centered/if its "portrait or landscape" oriented?

2 meter shot, ~ 0.2 m target objective.

That's ~ 10% spread from firing point to target to hit it....not too bad.

As mass is a lot of the score....the fewer parts the better, and as the throw has little recoil as it doesn't need to be hard...the firing platform can be minimal.

8. Dec 29, 2011

### Techcom0

Yes, that's why I'm trying to find ways to minimize mass as much as possible.
Will large foam wheels have a chance of getting to 1m in sand?
The height is around 5-10 cm, I believe. Nothing too outrageous.
Whereever the projectile first lands, that's where the final target hit counts as.

Oh yes, and it's landscape oriented.

9. Jan 3, 2012

### Techcom0

Any ideas for the delay mechanism to fire the projectile? ;/

10. Jan 4, 2012

### Danger

Welcome to PF, Techcom0.
I frequently get in trouble for answering things like this, because I tend to think very literally. If it isn't specifically forbidden, it's legal.
My first approach, therefore, would be to mount a Mac 11 on spring steel legs terminating in fat plastic tires. Place it at the top of a tall ramp, with a 1M string tied to the trigger. Drop and observe.
Assuming that your instructor would nix such an brilliant approach, I would suggest a barrage attack, with one honkin' rubber band firing 20 or 30 darts simultaneously in the general direction of the target. Again, I would use an anchored 1M string as the trigger.

11. Jan 4, 2012

### A.T.

Put a spool on the wheel axle that is winding up some string as the cart rolls. The other end of the string triggers the shot. The length of the string and the ratio of spool & wheel radii control after which travel distance it will fire.

12. Jan 4, 2012

### my_wan

By mass I assume the heavier the cart the better? Does launch position mean it is best if the cart stops at the point it fires?

For cart wheels I would use bicycle wheels. The larger wheel greatly reduce the lateral pull from the sand tracking a straighter line. A longer wheel base also helps track a straighter line. Then use a 5 gallon bucket of heavy material tied to a rope looped over a boom with a pulley and wrapped around a drive axle pulley like a yo-yo. Properly geared to insure a smooth slow propulsion. Now with the rope at a proper length the cart will automatically stop when the bucket hits a holding platform on the cart at a precisely measured distance. Now the heavy bucket can also be what releases the firing pin with a properly measured string.

The bicycle wheels do not have to use regular air tires so junkyard versions work fine. You can even tape water in place of the air tube and experiment with edges that track through the sand straighter under a heavier load.

For the projectile I would use a pvc pipe, a rubber band from a wrist rocket or one of those stretch tie downs, and a firing pin nearly as long and the pvc pipe, close to the same diameter, with an eye hook on one end. The ammo can be a steel ball bearing as large as it can be and still easily pass through the pvc pipe. You do not want the rubber band hitting the end of the pvp pipe until the bearing is ready to exit, else it might throw off the accuracy significantly. One end of the firing pin could be slightly magnetized to insure you didn't lose your bearing prematurely.

Mount the pvc pipe, then place the firing pin inside it held snugly inserted by the rubber band. The end of the firing pin that has the eye ring will stick out the back. The rubber band should be attached some distance from the pvc pipe at not too sharp and angle from the fired position to cut down on chaotic motion for accuracy. To cock pull the firing pin back and use a peg to lock it back. This is the peg that needs pulled to fire. Now as the heavy bucket drops propelling the cart forward it also pulls the firing pin via a string when the target distance is reached. Now just make sure the thing is properly tuned for the target. The whole thing should go off in one smooth roll fire motion like falling dominoes with very little sloppy motion.

(Duct tape is your friend, lots of it) :rofl:

13. Jan 4, 2012

### Danger

Gutsy fellow.
The only time that I ever tried to tape water, all that I got was a face-full of 'wet' and derisive laughter from the observers.

14. Jan 4, 2012

### Techcom0

No, the car has to be as LIGHT as possible. Gallons of stuff and bicycle wheels won't work. There's also the 20 cm x 20 cm limit for the car in terms of length and width. The height could be higher, but the car will have to support it with its small mass already.
I doubt the Mac 11 will be accepted. This is high school physics. Cool idea though!

15. Jan 4, 2012

### Danger

Bummer...

16. Jan 4, 2012

### my_wan

That was meant to be "water hose". :tongue: I actually used water hose to fix my flat on my bicycle when I was 8.

Oh, I misunderstood the weight issue on the score, got it backwards.

That means you can cut wheels from any flat material that doesn't flex too much. That way they can cut into the sand a little to get a truer track line, then glue a ridge around each side of the wheel to limit depth, increase traction, and limit friction. Don't know how deep the sand is going to be. Replace the the gravity drive with a rubber band motor. You still want a measured string wrapped around the axle that put a well defined travel limit on the cart and operates the firing pin. The gun would be close to the same except using a straw and stick instead of pvc with regular rubber bands. The straw can be glued to a stick for strength and stiffness while giving it a base to mount it with. Some kind of gun barrel is needed for accuracy. Like a pea shooter.

17. Jan 4, 2012

### Danger

Would they accept a semi-automatic version of the M11? :shy:

18. Mar 9, 2012

### wick

I was looking at different styles of cars. All sort of ideas form other counties and i thought of a car that looked cool. I am designing it to see what it will look like but this is the car design. take a mustang but where the wheels, brakes, and all the parts for the wheels. take propellers and put them where all for tires post to be. then take two on the side of the car so when you are hovering you can move. I no every one is reading that and thinking it's dangerous but make something around them so the blades are not able to cut you. Then take hydraulics and hook for propellers up so when in the car you can control it.

Then use the engine from a helicopter and put it in where the car motor is post to be at. Inside the car take the middle out of it and add all the gadgets for it to start and the lights everything needed in a helicopter. Then take the instrument panel and replace it with your speed, oil, gas, and other instrument.