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The swivelling chair

  1. Jun 30, 2007 #1

    I have a beginners question about physics. Let's say that you are sitting in one of those swivel chairs - one that can rotate back and forth (about the z-axis). And let's also say that you are sitting in the middle of the room, just out of reach of anything that you can grab on to. Your feet are on the chair, not touching the floor.

    Your goal is to turn around in this chair. My inborn sense of physics tells me that in order to accomplish that, I should hold my arms outward and to swing them in the direction opposite of which I want to turn toward.

    Question 1: What concept of physics explains the reason why this method works? (I studied first-year physics, so I should probably realize the concept if you name it).

    Question 2: I noticed that the further I try turn, the more difficult it gets. In fact, if I rotate 180 degrees and sit still for a while, and then try to turn another 180 degrees in the same direction, the chair will resist. It's almost as if the chair remembers my state of motion. As if the chair has a memory and a conscience... . Can anyone save my sanity before it's too late by explaining this phenomenon?

    Thank you in advance for the help.
  2. jcsd
  3. Jul 2, 2007 #2
    I believe you're asking that, if you're sitting in a swivel chair, if it is possible to make the chair rotate without touching anything.

    If we simplify the experiment to a frictionless environment with no air resistance, then the only way to rotate the chair would be to start rotating your body in the opposite way you want the chair to go.

    Because we are working in an environment where friction comes into play, this is not necessarily the case. If you jolt your body in one direction, you can make the chair move the opposite way (due to conservation of angular momentum), and then slowly move your body back to its original position while the static force of friction is not reached (the force required to start the chair into motion).

    About the chair "remembering" its previous positions, I think you just have a lopsided floor or something. :P
  4. Jul 2, 2007 #3
    A way I can think of in my head to do this (hold on let me try in my cubicle............)yes! conservation of angular momentum/energy, holds, what you do:

    sit in chair, put both arms out to your right
    sweep them in from of you at a good pace till they go straight out left. Then stop. Take your arms and lift them over your head till they stick out to the right, and repeat.

    Basically, your energy in moving your arms in a circular path will try to move your body/chair in the opposite direction. The same principle applies when you lift them over your head, but that axis of motion is locked by the wheels not being able to rotate into the floor, so nothing moves.
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