| MadSci Network: Physics |
You asked about what surface provides maximum speed for a matchbox car. My son suggested that the fastest surface is a vertical surface. He was correct, of course, but that was not the answer you wanted. To have speed and distance traveled a maximum, the frictional losses have to minimized. The surface that the car rolls on does contribute to some losses. One of these is hysteresis loss. If you pedaled a bike over soft soil, you know what this is like. To make hysteresis losses small, the rolling surface should be hard, smooth, and flat. Generally, the lower the deflection, the lower the loss. Some materials have lower hysteresis losses than others. High hysteresis materials (like soft soil or putty) have a dull, dead sound when hit. Low hysteresis materials (like steel or glass) ring like bells. If the material is hard enough, hysteresis losses will be insignificant. For example, railroad wheels on a steel rail have rolling frictional losses around 0.001. Automotive tires on concrete are ten to twenty times more than this. A bike on soft soil is much higher yet. Another source for loss occurs when the surface introduces steering error. An irregular, uneven surface may cause side slippage of one or more tires as they roll. If a channel is used to control the rolling path of the matchbox car, the channel shape is important. It should guide the wheel without introducing side slippage or vibrations. In brief, what you need is a hard, smooth, clean, dry, and non-sticky surface. Some references follow that you and your son may find useful. Pinewood derby racing http://www.worldforchrist.org/races/cars/why/pit.htm Rolling friction http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html Matchbox school project http://school.discovery.com/lessonplans/programs/frictioninourlives/
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