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How much energy can be generated by a swinging pendulum 60 kg

  1. Oct 27, 2015 #1

    RMM

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    I would like to make a pendulum go on moving by a permanent magnet that pushes the pendulum back with a force that is big enough to make it swing back with the same height. Can anyone tell me how much energy could be generated by a pendulum with height is 2 meter and 60 kg, swinging 90 degrees high on both sides. Imagine a swing in a theme park. How much Watt can I produce per sec (=Joule). If I go on swinging for one hour: can I multiply the energy by 3600? I used the formula
    E=mgh= mgl(1-cosA)=60*9.81*2(1-cos(45))= 558 Joule (Watt/s). That means over 2000 kWh? Probably I make a mistake :-)
     
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  3. Oct 27, 2015 #2

    BvU

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    Hi RMM,
    And how do you want to power this contraption ? You sure can store mechanical energy in such a thing, but if you take it out it's just hanging still and you need energy to get it going again. The magnet doesn't help you there !
     
  4. Oct 27, 2015 #3

    russ_watters

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    The amount of energy stored by a pendulum is just the potential energy at its highest point. So, do a little trigonometry to figure out that height difference and then apply the potential energy formula.

    That said, your description in the OP is a bit confusing because you first say you want to power it to keep it moving and then say you want to extract energy. These of course contradict each other....and smell like a desire for perpetual motion. Point being, you can only extract from it what you put into it (minus friction losses, which fortunately are pretty low with a pendulum).
     
  5. Oct 27, 2015 #4

    BvU

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    A few more comments/questions for you:
    • Joules are not Watts per second. On the contrary: Power is energy per unit of time. So Watts are Joules per second.
    • Where does your 45 degrees come from ? You only mention 90 degrees both sides. No trig needed: ##mg\Delta h = 1180## J.
    • Check your calculator too: ##60*9.81*2(1-\cos(45^\circ))= 345\ ## J
    • A pendulum with length l has period ##T = 2\pi \sqrt{l\over g}## s, so 2.8 seconds for your 2 m thingy !
    Back to the drawing board ?
     
  6. Oct 27, 2015 #5

    RMM

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    BvU: I indeed made a mistake in Watt= J/s I controlled the calc in Excel and the result is: 558 J (not 345: OK?) The period is indeed 2,8 sec and not 1 sec. I want the pendulum be pushed back from the highest point. See Walter Lewis in this video. From 25 minutes. I want to put a permanent magnet on the place where his head is to push the pendulum away. How much energy can I generate (take away) from the gravitational potential energy in a way that the pendulum does not stop.
    Mentor: How can I calculate how much energy the magnet must produce to push hard enough to come back at the same height and have a surplus on energy that I can take away (by transferring the rotation on the axis on which the pendulum is hanging) It sounds indeed like looking for a perpetual motion, but I want to put a permanent magnet for constantly adding energy. I saw the glass breaking in the video and I think, that there is a lot of energy that could be used.
    Thank you for your reply. I hope to get some more information, so I can go on or stop.
     
  7. Oct 27, 2015 #6

    russ_watters

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    The magnet can't re-generate the pendulum unless it, too, is moving. Otherwise it is producing a static force, which absorbs energy as the pendulum approaches and gives it back as the pendulum swings away.

    Also, glass is very brittle and therefore takes very little energy to break.

    Also, also, I'm still not clear on the swing angle, but if the 1180J is correct for the total energy available, you should consider just how little energy that is: it's on the order of the energy stored by a watch battery!
     
  8. Oct 27, 2015 #7

    BvU

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    In excel you have to present the angle to the cosine function in radians, not in degrees.
    60*9.81*2(1-cos(45)) = 558.791 but that is for 45 radians.

    ##\ 60*9.81*2(1-\cos(45^\circ))= 345\ ## J. It really is. And the ## 2(1-\cos(45^\circ)) \ ## shouldn't even be there. A simple ##mg\Delta h## is what you want. See also Russ' post.

    And a simple check: ##\cos(45^\circ) = {1\over 2} \sqrt 2 \approx 0.7071 ##

    And does the ball really break the glass (at 24' 17" ) ?
    (mind you: that's only the little bit of energy from his pushing instead of letting go!)

    And magnets usually attract iron, they don't push.

    Bottom line: all the energy you take out in whatever way is what you need to put in again with your pushing mechanism . In fact that is the message poor Walter is trying to get across.
     
  9. Oct 27, 2015 #8

    CWatters

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    That's difficult to calculate. It depends on things like frictional losses in the pivot and air resistance of the pendulum bob. Probably easiest to measure it. Release the pendulum from max height H measure how high it goes back up after one swing H1 then ΔH=H-H1 . The energy lost will be roughly Elost = mgΔh where m is the mass of the bob. That's roughly the amount of energy you will need to add per swing to make it come back up to height H each time.

    If you only add just enough energy to make it come back up to height H then there will be no surplus on the next swing that can be "taken out". You could add excess energy on one swing and take it out again on the next.... but it seems rather pointless.

    The maximum amount of excess energy you can add might depend on how much higher the pendulum can be allowed to go on the opposite side to the magnet. Too much and it goes "over the top".
     
  10. Oct 27, 2015 #9

    RMM

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    Russ: The glass breaks at 24.50. I saw other videos of the same demonstration with Walter Lewis and it really breaks. The force is the combined force of the push by Walter Lewis plus de gravitational kinetic energy. The gravitational energy alone is also painful strong as you would feel if you put your head in front of the glass after you let the ball go from the glass and put your head in front of the glass. Don’t try please!

    BvU: I got it. In Excel I changed the formula by changing the degrees in pi/180 per degree. So the formula is: =M*9,81* L*(1-COS(PI()/180*45)) That is 345 J. It really is!

    I have two egg-shaped permanent magnets as you can find HERE. If I try to put the same poles together, it is hardly impossible: the pushing away force is unbelievable strong. I will make a small scale try-out, perhaps with one egg at the left and the other on the right or I will use one egg as the bob.

    CWatters: just a little bit of energy will be “harvested”. The 345 J are set by a period of about 3 sec. So roughly 100 J per sec. Suppose that I can add by the magnets about 20 J (I have no idea how much Joule a magnet can add) and that I can use the 20 J per second to convert in to electric energy. BvU taught me: Watts are Joules per second. That means 20 Watts, right? In an hour I can make 3600 x 20 Watt = 72 kWh. That is: by 60 kg. Then I will need very strong magnets. If I change to a smaller scale: 1 kg bob, 2 meter L, swing is 45 degrees: I get 5,747 Joule per period of about 3 seconds. Suppose I can harvest 1 Joule per sec. I still produce in one hour: 3600 x 1 Watt = 3,6 kWh. That is as much as my Solar system on the roof with 3 solar panels of about 250 Watt produces in one sunny day. I think I still make a mistake, or we (RMM, Russ, BvU and CWatters) did find a revolutionary new energy device. What you think: shall I start to build one or are there other people who tried this?
     
  11. Oct 27, 2015 #10

    russ_watters

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    I understand all of that. What you don't seem to understand is that demonstration uses a tiny amount of energy.
    Ok....what you describe for a setup won't do that, but I'll go with it...
    No. A watt-hour is a watt for an hour. So 20 watts for an hour is 20 watt-hours or 0.02 kWh.
    Same error as above: 1 watt for an hour is 1 watt-hour or 0.001 kWh.
    At the very least you should be able to see that 250 watts is a whole lot bigger of a number than 20 watts or 1 watt.
    No, all you've discovered is that you misunderstand how energy works. However, I encourage you to try to build this device because it will help convince you that it won't generate any energy and should help you understand how energy actually works.
     
  12. Oct 27, 2015 #11

    RMM

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    Most important is for me to know that a Watt means: a Watt for an hour. That makes me understand, that the energy is much less than I did count on. Thanks again. Sorry for wasting your time.
     
  13. Oct 28, 2015 #12

    CWatters

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    Read up on the law of conservation of energy. You cannot "harvest" more energy than you put in. Sadly some people become obsessed trying to break this fundamental law of physics. It's so pointless trying that the subject is normally banned on this forum. Don't be surprised if this thread is closed.
     
  14. Oct 28, 2015 #13

    RMM

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    OK. It can be closed.
     
  15. Oct 28, 2015 #14

    BvU

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    My impatience, sorry.
    Gravitational force didn't change in that half minute ! So small push: glass stays, hard push glass breaks. No push (when he has his face in front): almost touching (meaning friction is pretty well negligible). Conclusion: Amount and force of push has to be enough to break the glass. No energy harvest posssible...

    But it was a challenging proposition. I enjoyed this thread !
     
  16. Oct 28, 2015 #15

    berkeman

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