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Homework Help: Inertial reference frames.

  1. Nov 4, 2011 #1
    An inertial frame is one which is not accelerating.
    i.e if I'm sitting in an accelerating bus or plane I'm not an inertial observer however if Im in a bus or train traveling at a constant velocity i.e zero acceleration then I am an inertial observer.

    One thing Id like to ask here is that if I have a constant speed but a non constant velocity that means I have a non constant acceleration. So that means I'm no longer an inertial observer correct?

    However If I stand on a turn table rotating at a constant speed.
    And if I place an object on the turn table with me. Then wont that object stay there? Or would that then depend on the mass of the object and the angular velocity of the turn table? In my opinion I would not be an inertial observer.

    One last thing I was going through Dr shankars( I believe) lecture of Yale on CM. And he used the example where he said that when you place an object on an accelerating plane it travels back. And that an object at rest did not remain at rest hence justifying that that is not an inertial frame. But doesn't the moving of the object backwards depend on its mass/inertia? Its not absolutely necessary for an object to move back right? Whats if its too heavy ie massive.

    - Thankyou, Jade
  2. jcsd
  3. Nov 4, 2011 #2


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    Staff: Mentor

    Yes. A non-constant velocity means that you are experiencing acceleration. Acceleration implies non-inertial frame of reference.
    Nope. It'll accelerate away from you (unless friction or some other force retards it). It will follow the dictates of the inertial forces that it feels in that frame of reference.
    From your point of view standing on the turntable, the forces that an object appears to experience will of course depend upon the angular velocity of the turntable.
    The heavier the mass, the larger the apparent inertial force. Making the mass larger does not change the outcome, in fact it makes it more certain! if the plane accelerates, it will leave anything not bolted down, or held in place by friction, behind. Remember, in reality it is the plane that is accelerating and the other objects that just want to stay as they are (Newton's 1st law!).
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