A common source of frustration when I'm at work is the fact that my rolling office chair's wheels rotate whenever I push it forward or backward from my desk, which can cause it to bump my computer tower or any other object on the floor nearby. This happens no matter how carefully I avoid turning my torso, or how straight back I push.
What physical phenomenon causes office chairs to rotate in this way? It has five wheels, if that's important.
My suspicion is that there is some interaction among the friction forces of the wheels on the ground that gives rise to this behavior, but I don't understand how to connect the dots from "opposite the force of the push" to "circularly about the support column".
This is different from other physics.se office chair questions like these three because I am asking about the rotation along the vertical axis of the wheelbase, not why the chair has five wheels or how it can move without my feet on the ground.
Best Answer
The problem is, that the bearings are not perfect. Especially the ones that are used to turn the "feet" of the chair. Those bearings are under a quite heavy load, and they are not designed to turn without friction.
As such, your chair's feet always want to remain in the same position as they are, and that is usually not exactly the direction into which you want to move your chair. Frequently, you cannot move into any direction without turning some feet.
So, when you push your chair back, the feet of the chairs basically hold a contest on who's the first to turn. Now, like all the feet, the ones that remain in position relative to the chair's base, are generally not aligned with your movement either, and they provide the chair's base with some rotating force.
You can make the test of aligning all five feet straight to the back, and then slowly drawing the chair back. You'll see that you'll be able to move the chair for some distance without the base turning. However, once the imperfections of the setup amplify, one or more feet become significantly out of direction, and that's when the chair's base will start turning.