The velocity timer stops the watch as the bicycle's front wheel reaches the second chalk line. The test pilot continues up to the third chalk line, then applies the brakes. The stopping - distance team then swings into action and measures the skid length or stopping distance. The stopping distance will be proportional to the energy required to stop the vehicle. Do this experiment four or more times for each bicyclist at each speed, providing the class with a perspective on variability in experimental data and the use of an average to calculate an experimental result.
Students can calculate the average stopping distance and average speeds for each bicyclist.
They can also plot individual trial run data on a graph of velocity versus stopping distance. The stopping - distance data for each test pilot should be labeled with the same symbol. The stopping distance should vary with the square of velocity for each test pilot where it is reasonable to assume mass does not change significantly between each run. Ask students to identify sources of experimental variation, such as differences in breaking points, braking pressure, and the surface friction. Experimental results will also be affected by the accuracy of the velocity timer and the stopping - distance measurement team.