# Center of mass for derby car

1. Jan 21, 2008

### bart Sherer

I am trying to determine the center of mass of a pinewood derby car. Any suggestions on how this can be done?

2. Jan 22, 2008

### mgb_phys

On paper, my measuring the shape and mass of each component - calculating their individual CofG then calculating the overall value.
Or just balance the finished car on a knife edge.

3. Jan 22, 2008

### bart Sherer

Thanks. There is also a lot of discussion as to where the weight in the car shoud be placed. I assume the weight should be placed as low as possible and as far back on the vehicle as possible. Your input is appreciated.

4. Jan 22, 2008

### Integral

Staff Emeritus
Hang the car from the middle of a string, the hanging (free) end of the string will pass through the cm. Do this from 2 different points, the intersection is your cm.

5. Jan 22, 2008

### Andy Resnick

To find the center of mass, you actually need *3* different 'hangings' because the car is 3-dimensional. Two will identify the line that passes through the center of mass, the third will reduce it to a point.

In any case, IIRC, the number 1 design consideration is the wheel friction. The wheels must spin freely and be lubricated. Then, the weight and aerodynamics come into play.