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Homework Help: Explanation for Non-Inertial Frames of Reference

  1. Apr 8, 2016 #1
    1. The problem statement, all variables and given/known data
    Why do objects that have no external net force acting on them accelerate? Ex. If a ball is on an accelerating train, it will accelerate opposite the direction of the train's acceleration, assuming there is nothing blocking its path of motion and it is not strapped down. My teacher says that we use the fictitious force to explain this motion because there is no net-force on the ball, but is this the actual reason?
    2. Relevant equations

    3. The attempt at a solution
    Does the ball accelerate in my given example because it wants to remain in the same position it was in before the train began accelerating. That's what I think, but this explanation doesn't make sense, because it uses inertia to explain the ball's motion, even though the motion goes against what Newton's First Law says anyways, because the ball is moving even though there;s no net force acting on it.
  2. jcsd
  3. Apr 8, 2016 #2
    the observer in the accelerated frame observes that a ball is moving so he measures the acceleration and finds a force - a pseudo force in words of an inertial observer as there is only motion of the frame and no agency is involved generating a force for the movement of the ball- a pseudo force is very much there for non inertial observers and newton's laws are valid if we apply a pseudo force to the body.
    in your daily experience on earth the 'centrifugal forces' are also pseudo force generated by accelerated motion of the frame-like centrifuge which we use in washing machines.
  4. Apr 9, 2016 #3


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    Let me give this one more try.

    First, you are very confused about what your teacher said and what is actually happening. So, you need to rewind and start thinking about this from the beginning.

    Let's consider this situation:

    There is a train at rest in a station with a ball in it. Probably best to think of a freight train with its doors open so you can see the ball. You are on the platform and there is another ball at your feet (at rest on the platform).

    The train begins to accelerate slowly. Everything that is attached to the train accelerates forward with the train. But, three things do not move:

    You don't move (because you're not on the train)
    The ball at your feet doesn't move (because it's not on the train)
    The ball on the train doesn't move (theoretically) because there's assumed to be no friction to pull it forward with the train.

    And, in fact, the platform doesn't move either!

    What you should see is the ball on the train and the ball at your feet stay exactly where they started and not move an inch.

    That's the first thing to try to understand.

    Now, suppose the train driver looks out the window, back over his shoulder. What does he see?

    He sees you accelerate away from him.
    He sees the ball at your feet accelerate away from him
    He sees the platform and the station buildings accelerate away from him
    He probably can't see the ball inside the train, but if he could, he would see that accelerate away from him.

    The train driver is in a non-inertial reference frame. He sees things with no forces on them accelerate away from him. And, if he looks forward, he might see trees and other things accelerate towards him.

    Finally, how does the train driver explain this?

    If he wants to use Newton's laws, then he has to invent "fictitious forces" to explain the motion he sees. From his view, there is a fictitious force on everything not fixed to the train in the opposite direction from the acceleration of the train.
  5. Apr 9, 2016 #4
    That makes sense, thank you/
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