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Why when swinging do you come away from your seat?

  1. Jun 5, 2012 #1
    This question has been at the back of my mind for ages now and it's decided to start poking at me for an answer.

    Why do you come away from the seat at the top of your swing? All I have is the forces acting on the whole system which would be SHM and circular motin but how are the forces acting on you as a person changing as you get to the point of maximum amplitude?
  2. jcsd
  3. Jun 5, 2012 #2
    As you swing the forces on your body are gravity (straight down) and the radial force of the swing which is due to the tension in the rope/chain of the swing, which keeps you moving more or less in a circular arc (constantly pulling inward toward the bar that the swing is attached to). At the apex of the swing right before you start to move down again you are momentarily in free fall, being acted on only by gravity. If someone cut the rope right at that point you would fall straight down, as your velocity relative to the Earth (and its gravitational field) is zero. Soon afterward you are pulled back into circular motion by the radial acceleration of the swing. Your body wants to keep moving in a straight line at each point along the semicircle, but the seat exerts a radial force that accelerates you inward along the arc of the swing. This is why you feel weightless at the top of the swing, the radial force that "pulls" you into your seat requires that you are moving around the circle, not at a standstill, which you are for a brief moment at the apex.
  4. Jun 5, 2012 #3
    Another way to think about it is the disappearance of the fictitious centrifugal force at the apex of your swing. Throughout the circular motion you experience centrifugal 'force' due to being in an accelerated reference frame. At the top of your swing, when you are no longer in motion (briefly) the centrifugal 'force' that kept you down into the swing vanishes.
  5. Jun 5, 2012 #4


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    This only happens if the amplitude of the swing results in the chain (or rope) being horizontal (or nearly so). On a low amplitude swing, there's very little vertical acceleration, so it's not a free fall situation.
  6. Jun 5, 2012 #5
    At the top of the swings motion, you and the swing both enter a free fall state due to gravity and a lack of tension in the chain. Once the tension in the chain is restored and the swing enters circular motion, friction keeps you planted in that seat once again.
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