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B Energy in Pendulums.

  1. Jul 26, 2018 #1
    Hi everyone, I'm new to this sight/forum so I apologies in advance if I've got the wrong platform or have made any other faux par.

    This is probably very elementary to most on this forum; can someone please tell me and hopefully explain a few things around transfer of energy in pendulums. Here is what I am pondering....

    Is the energy required to raise a pendulum bob to its point of maximum potential energy more, less or equal to the energy then seen in the movement of the bob back and forth through the resulting oscillations? - I realise that the exact measurements of this depends on the design of the pendulum and assumes a relatively low level of friction but I'm more or less just referring to a string with a spherical weight suspended one one end and fixed to a fulcrum point at the other.

    From my limited understanding of this matter I would think that the sum of the energy (mass X gravity X height) needed to send the bob flying past its point of equilibrium a number of times is collectively greater than the initial energy requirement to raise the bob from equilibrium to its point of maximum potential energy, this extra energy being drawn from earths gravity.

    Can someone please confirm if my understanding is correct of if not explain why and what is actually happening?


  2. jcsd
  3. Jul 26, 2018 #2

    Doc Al

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    The energy needed to raise the mass from the lowest point to a height h equals mgh. At the highest point, all the energy is gravitational potential. As it falls, that energy is transformed into kinetic energy. The total energy, ignoring friction and other losses, is conserved as the pendulum swings back and forth.
  4. Jul 27, 2018 #3


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    Its kinetic energy at the equilibrium position will, ideally, be exactly the same as the potential energy at the greatest displacement. The term "flying" seems to imply that it would be more??? It couldn't be, without a supply of extra energy.
    No different ideas here than in @Doc Al 's post - just put a different way.
  5. Jul 27, 2018 #4


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    Gravity can never be a "source" of energy. It's only ever a store of energy. A hydro electric dam stores energy but the source is the sun as it's the sun that provides the energy to raise the water/rain.

    Anyone claiming they have made a gravity powered machine is mistaken.
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