1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy thought experiment (oldschool very)

  1. May 14, 2013 #1
    This might seem a bit weird at first but it raised an interesting question to me about power, work and energy.
    I watched this show about how the trebuchet is so much more efficient than previous designs and then I sort of thought of it as a way to store energy and I wondered to myself, if you had one that was geared from the fall to be a speed multiplier connected to a 240V motor (so the long and short arms fell very slowely and the motor spun quite fast) "how long would it run for, 1 min, 1 hr?"
    (to be precise I mean like the rate of which it was falling is equal to that necessary to make the motor put out 240V, approximately no more or less)

    Then I realised I didn't know how much energy was stored in it. So I'd like to ask anyone, if you had a trebuchet with a mass in it (half a tonne, a tonne, whatever you want) and the height for the short arm to fall was 1m, 10m, 20m, whatever you like, and the lengths of the short and long arm were whatever you specified. How could you know how much energy was stored in it once the mass was raised to full potential (probably a bit over 90degrees) ?

    If anyone is interested enough, or knows already could you share?
  2. jcsd
  3. May 15, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The energy E stored in the trebuchet is simply...

    E = mgh

    m=mass of counterweight
    g = acceleration due to gravity
    h = height the counterweight falls (find using geometry).

    Not all of that energy goes into the projectile because the counter weight has KE at the bottom.

    The maths for this light is much the same...


    although there is a problem using gravity for lighting which I will let you figure out using the following...

    Energy = power * time


    power * time = mgh

    Pick a power for your light...say 40W
    Pick a run time...say 1 hour or 3600 seconds
    Pick a height for your lamp...say 1meter

    The required mass is ??
  4. May 15, 2013 #3
    Right'o, ok so say we're running 40W (friction etc ignored), then if our mass is 500kg, g = 9.8Nm^2, and the counterweight is 15m high then it will run for half an hour?

    500*9.8*15 / 40 = 1837.5 seconds = 30.635

    (meaning that there is 73500 N of energy stored in it? (40W*1837s))

    Nice article about the lamp btw, though I would have thought it would have made more sense to keep the energy mechanical rather than lighting LEDs with it.
  5. May 15, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Not exactly going to be a small desk lamp then :-)

    In short. Gravity is relatively a weak force.
  6. May 15, 2013 #5
    Well you say that, but if you engineered a skyscraper so that it could be ratcheted up a few inches over the day from wind and solar power you've got yourself a respectable battery. XD
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook