Science project

proj · Post by **proj** » Wed Mar 14, 2007 12:17 pm

I have been assigned a science project in which I have to build a self propelled car that travels at least 10 ft. I can only use the following materials: rubber bands, two blocks of styrofoam, aluminum foil, gloves, spool of thread, deck of playing cards, string and pencils. I tried to build a motor using the spool of thread and rubber bands. But it only travelled 2.5 ft on average, and it spun around. PLEASE HELP ME!!!!

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 12:33 pm

If it went straight instead of spinning would it have gone 10 feet?
How fast did it start?
Does it always spin in the same direction?
What did you make the wheels out of?
What surface ("pavement") are you running on?
Anything else you can tell us about how the powered and unpowered "wheels" interact with the "pavement"?

proj · Post by **proj** » Wed Mar 14, 2007 1:08 pm

I don't think it would've gone 10ft, but 5 for sure.
It started fast, but not too fast.
It always spins in the same direction, to the right.
The surface is concrete.
Sometimes the spool(wheel) spins, but it doesn't move. Most of the time it starts fast but then is moves to the right, or slows down even if the rubber band is wound up to the maximum.

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 1:20 pm

Sometimes the spool(wheel) spins

What material is the spool (wood, plastic, styrofoam, something else)?
How many wheels do you have? If both "rims" of a single spool are touching the floor, then count it as two.
What size spool are you using (approximate diameter and length)?

I'm asking these questions to try to understand everything that is in contact with the ground. The wheels "spinning and doesn't move" is a probably either a traction problem on the driven wheels or a friction problem on the non-driven wheels. The turning to the right is a control problem which maybe an axle alignment issue or a differential friction problem or something else.

Is your vehicle front or rear wheel drive?

proj · Post by **proj** » Wed Mar 14, 2007 1:29 pm

The material of the spool is plastic.
It has two wheels.
The length of the spool is approx. 2 in. and the diameter is approx. 1.5 in.

The vehicle is rear wheel drive.

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 1:55 pm

Two wheels or two spools? What is holding up the front of the vehicle?

Are you limited in how many rubber bands or what sizes you can use? If not, your traction problem on the drive spool might be solved by using a wide rubber band as a "tire" on each "rim".

Are you using pencils as axles? If so, are they the hexagonal or round variety?

The material list doesn't seem to have any "glue" or "tape" so how are you holding the pieces together?

proj · Post by **proj** » Wed Mar 14, 2007 2:08 pm

If both "rims" of a single spool are touching the floor, then count it as two.

It is one spool, with two rims. It is made based on this model:
http://www.pbs.org/saf/1103/teaching/teaching3.htm

There is no limit as to how many rubber bands I can use.
Glue and tape can be used.

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 2:51 pm

In the reference you cited, are you dealing with just "Part 1" (the motor) or do you have a "Part 2" vehicle?

The weight distribution and balance of the "Part 1" motor only is prabably not going to go straight by itself. I'm curious what you are using as a replacement for the "cotton swab"? What ever it is, one end has to be dragging on the floor unless you have a "Part 2" vehicle. It is also acting as a lever and applying some vertical force (up or down) to that end of the spool. This drag, the vertical force, and the length of the spool, and the fact that the torque is applied at the opposite end will cause the turning.

proj · Post by **proj** » Wed Mar 14, 2007 3:27 pm

I am dealing with part 1.

Thanks to you, now i know why it has been moving sideways. But wouldn't it happen even if we are dealing with part 2, since torque is applied at the opposite end? What to I have to do in order to make it move a long distance?

The replacement for the cotton swab is a pencil.

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 4:18 pm

When you get to "Part 2", the pencil that is now acting like a rudder and lever on the floor should be attached to the "chasis" so it will no longer be applying a vertical force (up or down) with respect to the floor and will no longer be dragging.

Any torque or any remaining vertical force can be compensated for by the placement of a weight at an appropriate place on the "chasis" if needed.

deleted-71588 · Post by **deleted-71588** » Wed Mar 14, 2007 4:27 pm

Oops, forgot to answer:

What to I have to do in order to make it move a long distance?

Why don't you wait and see what happens in "Part 2" when you eliminate the pencil drag? Because you aren't limited to the number of rubber bands, you can get more torque if you need it, but you don't want to create too much and make holding "Part 2" together a big problem to solve if you don't have to. I'm guessing that what you do in "Part 2" will eliminated drag, make it go straight, increase traction, and otherwise change a lot of variables that will affect how straight and far it will travel.

proj · Post by **proj** » Wed Mar 14, 2007 4:33 pm

Thank you very much, Craig.