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Centrifugal force and artificial gravity

  1. Dec 14, 2012 #1
    In lecture 5 of MIT's OCW 8.01 , Prof. Lewin says that when you're in in uniform circular motion, the perceived gravity is always in the opposite direction as the push or pull (so if you're holding onto the end of a rope that's being spun around horziontally, the rope is pulling on you and you feel gravity in the opposite direction).

    My question is does this perceived gravity have anything to do with Newton's 3rd law? If the rope is pulling on you then you're pulling on the rope as well?

    And is this 'perceived gravity' due to what's called centrifugal force? How does it work and why does it make you feel as though gravity's pulling on you? Doesn't the centripetal force point inwards?
     
  2. jcsd
  3. Dec 14, 2012 #2
    Compare holding on a rope hanging in strong gravity to holding on a rope being swung around a pivot. In the first case, you and the rope must supply a force to keep you from falling down. Your grip on the rope has to hold your weight, so you are pulling yourself upwards to counteract gravity which is pulling you down. The rope is also pulling you up. In the second case, you and the rope must supply a force to keep pulling you inward, toward the pivot. But you feel a fictitious force, centrifugal force, pulling you outward from the pivot, so you must pull on the rope to keep yourself on the rope.

    Centripetal force points inward. You and the rope supply the centripetal force.

    In an inertial frame of reference, centrifugal force doesn't exist, and centripetal force points inward, pulling you in a circle. In a rotating frame of reference, centrifugal force points outward, and centripetal force points inward, canceling out the forces on your body, so you don't move (relative to the rotating frame).
     
  4. Dec 16, 2012 #3
    Why do ou feel the force pulling you outward?
     
  5. Dec 16, 2012 #4
    Without any forces, you would move in a straight line, which would take you away from the pivot. Thus, inertia is the source of this centrifugal force. You have to fight inertia to stay a fixed distance from the pivot, hence it feels like a force is pulling you away.
     
  6. Dec 23, 2012 #5
    Ok...let's see if I understand this...

    Say you're on some sort of giant turntable with a wall. If the friction between the floor and you isn't enough to provide the centripetal force to keep you turning with the turntable, then you slide and hit the wall. Now the wall is pushing against you to provide the centripetal acceleration, and the reaction force of you against the wall is the force you feel - the 'artificial gravity'? Is that right?

    And for the ball and the rope, the rope pulls on the rope and so the reaction force in the non-inertial frame is the ball pulling back on the rope, and it's this pulling is the gravity that the ball feels

    Also, in the non-intertial frame, you don't think you are accelerating do you? You just feel a force pushing you outwards if the friction between you and the floor isn't enough to keep you going in a circle
     
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