Science Buddies: "Ask an Expert"

I am assisting my son with the construction of a "work force" car powered by no more than 3 rubber bands. It is premised off the equation of "W=mad." The student whose vehicle that travels the farthest on a tiled classroom floor will win. I suppose the vehicle will be heavy ("mass") and the rubber bands from which "acceleration" will be derived probably be of the "heavy duty" sort. With respect to friction, the tires should have a proper balance as to grip the floor enough to keep them from spinning in place but not too much friction as to cause it to drag and lose momentum.

Any ideas on the construction of such a vehicle and where to obtain supplies will be greatly appreciated. This project is due in about a week.

Hi joman401.

Is your Son allowed to make a "mousetrap car?"
This is a popular Project.

Do a Search on Science Buddies here for keyword
car

Do some Google Searches using keywords like this:
"car science project"

Here is one good Search Result:
http://www.tk.k12.mi.us/page/wissink/Mo ... roject.htm

Here is a Google Search Result for keyword:
"formula w=mad"

http://mchi.mcallenisd.org/www/mchi/sit ... emhelp.htm

I hope this helps with your Son's Project.

Good Luck!

Hi Joman401,

The friction that keeps the tires from spinning in place is different from the friction that will cause drag on the car. In the first case, you're sliding the tire across the floor, in the second case the point of contact between tire and floor doesn't move and all you're doing is bringing the two together face to face and flexing the tire a bit.

My guess is that so long as you don't make the tires out of something intentionally sticky, you probably won't have to worry about tire friction slowing your car. Friction in the axels is probably a much bigger concern.

Most large art/crafts stores sell suitable materials - blocks of balsa wood and coat hanger wire are likely candidates. Hardware stores or hobbies / model-airplane stores will also carry what you need.


Best,
Erik

Actually you WANT tire friction. The better a connection between the wheel and the road surface, the more power you can put on the wheel without causing the tire to spin.

The friction that hurts you is that between the axle and the body of the car, and possibly air resistance if you have a light car with really non-aerodynamic body.

There are several other people on this forum doing car experiments and these topics have been covered elsewhere, but from my own experiences designing cars that run from rubber bands:

1. Most of the power in the rubberband is released right at the beginning - which means first the tires spin and then they go for a little while and then the car just rolls, unless you work to try to get the power to go a bit more evenly. Our task included climbing a hill halflway down the path, so we had to change the gear ratios in order to get any power after the first few feet. Barring something like that, it is ok to just try to get a burst of power at the beginning and let the car coast, but you may discover that a heavier car actually goes further, because you can torque more power in the wheels without making them spin.

2. Large wheels minimize variation in where the car goes after it is released. If the accuracy of the car's travel is important, you may find a big, clunky design with huge wheels works better than a tiny, finely crafted design with small wheels.

3. Having 3 rubberbands gives you more flexibility than we had. You might be able to rig the thing to have three "power points" where a new rubber band "takes over" after an old one is exhausted. That is going to take a fairly sophisticated design, even moreso than putting in gear ratios to get the most out of a single rubberband.

A lot depends on how success is defined for your car, how you'd design it.

Ah...you only have a week.

Ok, simple design. Get a square box for the frame of the car - plexiglas is easy to work with, but wood or even aluminium is also fine - whatever you can knock together quickly.

It has to be fairly sturdy, because 3 rubberbands will exert a fair amount of force when extended.

Drill holes and put two axles through the box, attach the wheels on the rear axle firmly to the axle (if the axle spins, the wheels should spin).

Do what you can to make the axle spin freely. If you can actually find some bearings, that's ideal, but you could just drill a smooth hole and grease it too.

Rig some way of attaching a rubberband to the front of the car, and to the power axle. Wind up the rubberband by rolling the car backward. Observe where the "Friction point" is, where the wheels spin. That's what you're trying to optomize.

If you do this much, you'll have an entry. Now is the time to fiddle with it to make it better. You will probably be experimenting right up till you have to test it out

You can add more power by adding weight directly over the wheels. You can add power by thickening the axle in places, and having the rubber band start right on the axle, but as it relaxes having it turn the thicker and thicker parts of the axle (getting the same torque as your "friction point" where the wheels spin several times as it relaxes).

There are a lot of tricky things you can do with 3 rubber bands, and I encourage you to experiment, but keep in mind the two big limits:

1. Too much power and your wheels spin.

2. Each rubberband releases most of its power right as it starts to relax.

Have fun with it. But first get something that "works at all" before trying to make it "as good as possible".