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Re: [Phys-l] Pinewood Derby Car Weight....



Even in a model without air resistance and without any friction at the axles, a fraction of the PE decrease has to go to rotational KE of the wheels. As you increase the mass of the cart body (without changing the wheels), the final translational KE constitutes a larger percentage of the total KE. Put another way, a larger percentage of the PE goes to translational KE when the cart body is more massive, resulting in a faster cart. That may be why it pays to screw up your cart so that you only have three wheels that need to be rotated instead of four.

At our pinewood derby, we had an extra "cheater's race", where anything was allowed provided it didn't pose a threat to the facilities. Out of curiosity I entered a Pasco dynamics cart (slightly modified to increase the amount of clearance under the body). Its time was about a factor of two faster than the average.

Richard Zajac
Kansas State University at Salina