Toys > Cleaning

I was cleaning up around my house the other day because it was getting to the point where I had to climb over stuff to walk through the living room, and I found a toy I received as a gift a few years ago. Since cleaning isn’t exactly fun, I decided I’d play around a bit with this toy. This toy consists of a wheel with magnetic tips that rolls along two metal rods. The wheel is held to the rods because of ferromagnetism. When you place the wheel on the rods and tilt it, the wheel will begin to roll along the path of the rods (see video 1).



Tilting is required to move the wheel because the force from the weight (mass*gravity)needed to overcome the inertia, or tendency to resist changes in motion, of the wheel. Once in motion, the wheel accelerates until it hits the bottom point of the rod. Assuming I started the roll from the top of the rod (holding the rod perpendicular to the ground), the wheel will have a potential energy equal to the mass of the wheel multiplied by the height of the rod. Because of conservation of energy, the kinetic energy of the wheel while at the bottom of the rod will therefore be equal to the potential energy of the wheel while it was at the top of the rod. In an ideal situation, the wheel would then be able to rise to the same point it started after it turns around for an upward motion. My wheel does not rise to the same level again at the second try, however (see video 2). This does not disobey the conservation of energy because energy was taken away from the system by friction (there is rotational friction between the wheel’s tips and the rods).



Because mechanical energy decreases due to friction each time wheel goes back up, the wheel reaches a progressively lower height. To keep the wheel moving, you must keep tilting the rod in a way that allows gravity to provide force to the wheel to replace the energy lost from friction. The lower you tilt the rod, the greater the y-component of force from weight, and thus the greater the speed of the wheel. As you can tell from the video, I was so blown away by the sheer awesomeness of this toy that I didn't bother to finish cleaning the house. I'll finish that...soon.