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Frame of reference and Pseudo forces

  1. Jan 9, 2015 #1
    If you have a car and a pendulum is hanged inside it, and if you watch from outside the car,and if the car has some acceleration, then you see the bob moving in opposite direction of motion of car. For a person standing outside, he knows that the car has some acceleration. What causes the bob to move backwards from his point of view?
    its not pseudo force because the frame of reference is inertial.
    the car accelerates and so does tge bob. But why in opposite direction?

    Also if you have a person inside the car, since bob is at rest, he introducea the concept of psuedo force to balance tension and weight. Am I right?
    How do you explain first case?
     
  2. jcsd
  3. Jan 9, 2015 #2

    Doc Al

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    Staff: Mentor

    From the inertial observer's viewpoint, the bob is not moving backwards but is being dragged forwards by the tension of the bob. Since it is accelerating, you can deduce the angle that the pendulum makes.
     
  4. Jan 9, 2015 #3

    ShayanJ

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    Gold Member

    You're wrong. For someone outside the car(who knows the car is accelerated), the bob is just trying to stay at rest, but the string pulls it along with the car. So to avoid the complication caused by the string, lets just say there is a big glass container that can move with non-zero acceleration. We also assume that there is mass inside this container that has no friction with the ground. Then if the container accelerates, the mass just remains at rest and people outside the container, just say container is accelerated and the mass is at rest. But people inside the container(who don't know the container is moving) see that the mass is moving with some acceleration. So they say there is a force acting on it.
     
  5. Jan 9, 2015 #4

    jbriggs444

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    The bob does not move backwards from the outside point of view. The car moves forward.
    The car accelerates. The bob does not accelerate. At least not until the pendulum's angle has changed and tension increased, causing it to do so.

    For the person inside the car, tension and "weight" are not balanced. The weight includes a pseudo-force component that causes a net motion. The concept of the pseudo force is introduced so that Newton's second law is consistent with the observed motion.
     
  6. Jan 9, 2015 #5

    Doc Al

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    Yes. From the non-inertial frame of the car, a pseudo force is introduced so that Newton's 2nd law can be used. Since the acceleration is zero (assuming it is in its final position, at an angle with the vertical), the net force, including the pseudo force, will be zero.
     
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