Bicycle Wheel and Metal Rotating platform
This demonstration illustrate conservation of angular momentum along the z-direction. It requires a bicycle wheel with handlesand a rotating platform. Typically this demo is a hands-on exercise for students in physics 121 lab. Various initial conditions are possible: the simplest one is to start with the angular momentum confined to the wheel and pointing in the z-direction. Keep the platform from rotating while spinning the bicycle wheel, step onto the platform and held the bicycle wheel over head with its axis aligned with vertical. Once this initial state is set, one can tilt the wheel axis in any direction and discuss the resultant rotation of the system (wheel + person). Although the combined moment of inertia(wheel + person) is slightly different from the initial configuration this should not affect much the results. An interesting variation, starting from the same configuration, is simply stopping the wheel. \n A different initial condition is to start with the angular momentum on the horizontal plane, say with the angular momentum perpendicular to our chest. In this configuration we grab, with both hands, one side of the wheel axis. Starting from this initial configuration, we can tilt the axis toward the vertical, either by bringing the wheel up or down. Going from the initial to the final state in this case, will change significantly the combined moment of inertia. Hence if will be difficult to disentangle the effects. A slight variation that keeps the combined inertia close to its initial value and the angular momentum still on the horizontal plane is to start with the angular momentum parallel to our chest. In this case we still hold the wheel with both hands, but we grab the wheel on both sides of the axis. \n Then, we just tilt the axes to point the angular momentum up or down. Starting with the angular momentum on the horizontal plane allow us to talk about why the horizontal component is not conserved and how this relates to an external torque. In all these cases, it may be better to start by asking the students to predict the resultant rotation based conservation of angular momentum and then perform the demo. The student should be encourage to describe the resultant rotation in terms of the angular velocity vector . If you have done the
bicycle wheel (d0017) demo,then they know how to get the direction and sense of the angular velocity vector for a rotating object. Please feel free to experiment, there are many more possibilities for the initial conditions and hence to have fun with this equipment.
