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Are you in inertial frames?

  1. Nov 7, 2005 #1
    Are you in inertial frames???

    hi, today my friend ask me 2 simple questions, too shame i cannot answer him...so i post it here:

    is the classroom an inertial system ?? earth is rotating around the sun - so ...can still consider as inertial ?

    2. If u working in laboratory to find so physics equation.. How you decide whether ur lab is an inertial system ( Coz if Lab is not an inertial system - then Newton's Law cannot apply)
  2. jcsd
  3. Nov 7, 2005 #2


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    Your classroom is not an inertial frame for the reasons you cite. Depending on the conditions of the physical experiment, you may or may not approximate your lab as being an inertial frame of reference. For example, if you are doing a Foucault pendulum experiment you must take into account the fact that you're not in an inertial frame. However, if you are doing ballistic pendulum experiment where the time scales are short compared with the rotational period of the Earth then you can safely ignore noninertial effects due to the rotation.
  4. Nov 8, 2005 #3


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    The presence of gravity makes any frame on the earth non-inertial. However, Newton's laws still hold.
  5. Nov 9, 2005 #4
    but his question is : How to determine that your lab is in inertial frame or not ?
  6. Nov 9, 2005 #5
    An inertial frame can be determined only by comparing it to a frame of reference. For example, I am in an inertial frame with respect to the earth but NOT with respect to the sun. The reason is that I am travelling at the same velocity as the earth so if I stand on a point and look at a point on the earth (such as my computer) it appears to be "inert" (not moving). If I look at the sun in the sky (not recommended), it would appear to be moving while I stand still because our relative velocities are not the same. An easier example would be if two cars travel at the same speed. Passengers in one car would view the people in the other car as sitting there and not moving (even though both cars ARE moving). If a person sitting on the side of the road watched the people in a car move by they might say they are moving at 40m/s (relative to the person on the side of the road), but to the people in the car next to them it would appear that the people are moving at 0 m/s (relative to them, they are travelling at the exact same speed so it appears that they are not moving).

    Basically, NOTHING in the universe is inert (everything is moving relative to something else). The "inertial frame" is an arbitrarily determined frame of reference with which we consider our movement to be zero while we measure the movement of something else (what we are actually measuring is therefore the *difference* between two velocities, and we just decide to start measuring by setting our velocity to "zero" as a reference point to start the measurement). This is all stuff credited to Einstein's general/special relativity if you want to read up on it.

    To answer the question, if you consider your current velocity to be zero and you look at the lab and the lab is not moving then the lab is in an inertial frame of reference. If you start running and look at the lab, the lab will appear to be "moving" at some velocity. So if you set YOUR velocity to zero as the inertial reference while running, then the lab is not inert (you can't set it's velocity at zero when you start measuring), because it is moving with some velocity relative to yourself.
    Last edited: Nov 9, 2005
  7. Nov 10, 2005 #6
    Thanks for the reply -- i am clear now
  8. Nov 11, 2005 #7
  9. Nov 12, 2005 #8
    That is true. I gave a "simplified" explanation to make the idea easier to understand. The inertial frame can exist for bodies "at rest" or moving "uniformly at constant velocity" (but not for other velocities that are not moving uniformly), but I have found it is much easier to explain it to someone with the condition that the bodies be at rest relative to one another for inertial frames (and that non-inertial frames be moving). That way they can see the distinction clearly and can add in the detail that constant velocity frames can be considered inertial at a later point (after they have grasped the basic concept).

    If I wanted to be a stickler, I can add more detail to the car example by saying that both cars were accelerating at the same rate relative to each other (that is, their velocity is changing every second), and that the person on the side of the road measured an instantaneous velocity of 40 m/s and that the two cars are in inertial frames with each other, whereas the guy on the side of the road is not in an inertial frame with respect to either car, but this attempt at being "more correct" tends to confuse things, no?
  10. Nov 12, 2005 #9
    i spose that would make it confusing...
  11. Nov 12, 2005 #10

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    An inertial reference frame in classical physic is one in Newton's Laws are valid. The Foucault Pendulum shows that F=ma is not true on the surface of the earth -- i.e., the earth is not an inertial reference frame. No comparison to other frames need be made.
    Last edited: Nov 12, 2005
  12. Nov 12, 2005 #11


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    An "inertial frame" is not determined arbitrarily. It can be operationally defined by the state of an accelerometer (and Foucault pendulum) carried by the frame, without reference to any external reference frame. This determination is not "relative"... it is "absolute".
    Consider two identically-moving cars traveling alongside each other. Suppose they decide to throw a ball from one car to the other. If they are travelling inertially, then the drivers will do exactly the same thing they would if they were at rest. However, if they are "accelerating at the same rate relative to each other", then one will have to throw the ball differently (because the ball will travel inertially). This unambiguously distinguishes the inertial case from the non-inertial one...without regard to a third reference frame.
  13. Nov 14, 2005 #12
    There are two ways to answer this. Newton would answer it one way and Einstein another way. Newton would say that you're in an inertial frame, but that there are gravitational forces present. Einstein would say that its impossible to determine if you're at rest (i.e. sitting in a chair on the earth's surface) in a uniform gravitational field or at rest (sitting in the chair of a rocket) in a uniformly accelerating frame of reference.

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