# Homework Help: Physics tournament

1. Oct 18, 2004

### ponjavic

Now it's time for me and a friend to prepare our project for a physics tournament in school.
Construct a car which is proppelled solely by wind energy. The car should be able to drive !straight! into the wind. Determine the efficiency of the car.
I would like some ideas for an efficient solution. I have researched how boats do it. They can't sail straight into the wind they can only zig zag, which is not sufficient.
My idea was to use bernouli's principle which is the one that aircrafts use. Create a low pressure in the front of the car and hight pressure behind it, this might be able to move it forward.
My friends idea was to use, i dont know what you call it... umm it consists of three cups

,,,,,,,,,,<
,,,,,,,,,,|
,,,,,,,,,/ \
,,,,,,,>,,,/\

Something like that, it's three cups placed with angles of 60 degrees. This module always rotates no matter from which direction the wind is coming. This might be used to drive some kind of axel which then moves the wheels...

Any ideas or suggestions?

We are in high school, but the project is supposed to be solved with help of others, we are to consult with professors later on. Our task is mainly to make a final presentation of our ideas and lab results and then to present and defend them.

2. Oct 18, 2004

### NateTG

Looks like a good idea. In practice, it may be easier for you to use four instead of three cups. Bernouli's principle won't allow you to go into the wind.

If you will only be going into the wind, you could mount the windmill horizontally, and spare yourself the trouble of building an angle drive.

3. Oct 19, 2004

### ponjavic

For the windmill that was the idea, could you explain why bernoulli's principle won't let me go into the wind?

Guess we'll try building the windmill then, less we find another solution.

4. Oct 19, 2004

### NateTG

First off, it's not entirely correct that you could not use bernouli's principle - but you'd end up building a windmill, and the cupped version is more efficient.

The bernoulli principle generates a force in a direction perpendicular to the airflow. Going into the airflow, the beroulli principle won't provide any force in the direction you want to travel.

5. Oct 19, 2004

### ponjavic

Damnit! :P
Yup i see now how the windmill is using bernoulli's principle.
What do you think about vertical and horizontal windmills?

Could you explain why i cannot create a low pressure point in front of the car and a high one behind it. I understand that i cannot but why?

6. Oct 19, 2004

### NateTG

Well, there's lots of ways to think about it. Here's a rather simple one:

Conservation of momentum -- your car is going to be slowing down the wind that hits it. That means that something is going to have to 'eat' the mometum. Since the car needs to forward, the only real choice is the ground. That means that any purely aerodynamic approach is going to fail.