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Power of the Pendulum - Proof of Ultra-Efficiency?

  1. Jun 3, 2012 #1
    "Power of the Pendulum - Proof of Ultra-Efficiency?"

    This video could be shorter, but if you have extra time on your hands take a look.

    He's got a pendulum mounted on a stand that is not fixed to the table. When he blocks the pendulum from swinging and strikes the apparatus the whole moves a small amount.

    But then when he unblocks the pendulum and strikes the pendulum, which then strikes the upright, the whole moves by a much larger amount.

    It doesn't make sense to me that transferring energy through a pendulum would be so much more efficient than a direct blow to the apparatus.

    To do the striking he employes a thing termed "measuring instrument with piston mechanism". I have never seen one of these, but the implication is that this instrument limits the amount of energy that can be imparted with it to a fixed amount, regardless of the number and force of the blows you make with it. The piston, it's implied, will "sum" the energy, coming to a stop at the end of the cylinder when the limit is reached. That implication strikes me as the erroneous one. If the air is escaping through a small orifice (it's not clear to me if it is) then the harder you hit with the thing, the more it will act like a spring. The air would just compress, then expand, driving the piston back out. He seems to be hitting much harder in the second set up.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Jun 3, 2012 #2
    Re: "Power of the Pendulum - Proof of Ultra-Efficiency?"

    With the pendulum blocked, he hits the ball ten times. With it unblocked he hits it five times the first round, and 6 the second round.

    You could try and push a large boulder ten trillion times without budging it, yet move the same boulder with one push a fraction of the net force of the ten trillion other pushes.

    I also wondered if the swinging of the pendulum may have an effect on the stand. When the ball moves in one direction, it might tend to change the weight distribution such that the weight is more concentrated on a smaller part of the base which might decrease the overall friction.
  4. Jun 3, 2012 #3


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    Re: "Power of the Pendulum - Proof of Ultra-Efficiency?"

    It's all in the "measuring instrument with piston mechanism"

    When he strikes the ball with the block raised, there is sufficient inertia resistance to push the piston in by degrees, thus imparting much energy to that, rather than moving the whole contraption across the table.

    When he removes the block, that inertal resistance / friction is removed, and much more energy is imparted to the whole contraption - notice how he 'follows through', just like you would with a snooker cue to make the white ball have much more momentum ?

    That momentum is translated to movement of the whole contraption when the metal ball eventually hits the vertical riser.

    Duh ..
  5. Jun 3, 2012 #4
    Re: "Power of the Pendulum - Proof of Ultra-Efficiency?"

    Yea this is nonsense, to do this objectively you'd have to accurately measure the amount of energy transferred to the system, and the one with pendulum unblocked will move less because some of the energy is stored in the motion of the pendulum.
  6. Jun 10, 2012 #5
    Re: "Power of the Pendulum - Proof of Ultra-Efficiency?"

    My thoughts - a swinging pendulum makes the stand unstable, meaning when it is in partial tilt there is only a fraction of the friction with the table. Pretty terribly designed experiment.
  7. Jun 11, 2012 #6
    Re: "Power of the Pendulum - Proof of Ultra-Efficiency?"

    Assuming that piston thing works, it will make sure the total amounf of momentum transferred is the same. The amount of energy transferred depends linearly on the velocity (positive if it's moving away), so there will be much more in the non-blocked case.

    If you have a pendulum that swings freely on one side, but bumps elastically into something halfway in its swing on the other side, the average horizontal force on it over an entire swing is of course 0, but there will be a large force when the pendulum hits the stand, and only a small force at all other times,
    (from the cord that the pendulum is hanging from)

    If the stand can slide over the table, the force is probably only large enough to overcome friction when the pendulum bumps into it.

    The extra energy from the piston is needed to keep this going, because the collision with the stand is of course not perfectly elastic.
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