The halfpipe used by skateboarders and BMX bike riders allows for some spectacular tricks. Given that the riders can rise nearly two meters above the top of the pipe, which is itself about three meters high, the forces and accelerations must be fairly large. It is the magnitude and timing of those forces that we studied.
The Discovery Channel held a competition for middle-school-aged kids who won their regional science fairs. The kids went through several challenges, among them the skateboard physics task. The question was simple: where, during the ride, is the halfpipe pushing on the rider the most, and where is the force the least?
To make these measurements, the riders carried a Vernier 3-Axis Accelerometer connected to a Vernier LabPro. Logger Pro 3 was used to configure the LabPro to perform remote data collection for 30 seconds. A standard video camera with a video capture board provided images to synchronize with the acceleration data.
In Logger Pro, I created a calculated column that is the square root of the sum of the squares of the individual acceleration values, yielding the net or scalar acceleration values. Scalar acceleration with an accelerometer corresponds to the perceived g-factor.
Any use of an accelerometer requires a short interpretation of the measurements: Because the accelerometer responds to both kinematic acceleration and the Earth’s gravitational field, the scalar “acceleration” is 9.8 m/s2 when the device is at rest. The measurement is really the Normal Force per Unit Mass, which we’ll call the g-factor for short. Kinesthetically, the g-factor corresponds to the compression one feels in the legs during snowboarding. You feel a g-factor of 9.8 m/s2 (or 1 g) when standing still.
This g-factor measurement is exactly what we needed to confirm the student predictions as to where in the halfpipe the forces were large, and where they were small. You’ll need the skater terminology: the halfpipe includes the vert, or vertical wall; the coping, or the railing top of the vert; the transition, which is the curved part of the pipe; and the floor, which is the flat part at the bottom. What is your prediction?
Here’s a video of the Logger Pro screen, showing both the video of a BMX biker, and the synchronized accelerometer data. Was your prediction correct?
This halfpipe physics report might remind you of another innovative use report, when I took an accelerometer with me on a snowboarding trip. Maybe I’ll have the nerve to try my board in a snow halfpipe this winter.
John Gastineau, Vernier Staff Scientist