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!

Hi, I am currently doing a project to design a lamp and am really

  1. Feb 2, 2012 #1
    Hi, I am currently doing a project to design a lamp and am really interested in using physics to gain a perfectly balanced design. I have come across this diagram...


    I just want to clarify a couple of things before I go ahead and produce it! Apologies if these are stupid questions but it is a while since I did any physics.

    1. I can turn these into any numbers as long as the ratio says the same right? E.g. Half the weights.

    2. The the pivot points need to remain exactly there in order to balance or does another option exist? By this I mean the lamp head is moveable in almost any direction, but with the layout of the arms in a different way?

    3. The measurements on the diagram (10,20 etc.) seem to show the measurement to the centre point of the weights... Is this what I need to measure to (the centre of mass of the weight?) or can the weights differ in dimension as long as the mass remains the same?

    4. Imagine a curtain pole (the arm) with a screw (the weight) half screwed into the end... Where would I measure 10,20 lengths to? The end of the pole or up to the start of the screw?

    Thank you so much for this.

  2. jcsd
  3. Feb 3, 2012 #2


    User Avatar
    Gold Member

    Re: Counter-weights

    The design assumes the weight of the arms is negligible compared to the weights.

    The 200 and 100 lamp works like this : 200 x 10 = 20 x 100 , for it to balance at the pivot. You may want to have a way to move the 200 weight on the arm a bit in or out to get a horizontal balance.

    Same thing for the 600 weight, so that when the top arm is pointing into/outof the page the second arm will balance horizontally. ie 300 x 20 = 600 x 10

    question 1 - as long as the weight of the arm is much lighter than the weights, so choose appropriate material for the arms.

    question 2 - not really sure what you are asking

    question 3 - center of mass of the weight to the pivot

    Question 4 - a heavy screw, much heavier than the curtain pole, measure to the centre of mass of the screw.

    You will have to make some small adjustments, as I said previously, to the lengths or weights for the thing to operate as in the picture.
  4. Feb 5, 2012 #3
    Re: Counter-weights

    Thank you so much that is brilliant. Thank you also to to the other replier who doesn't appear to be showing up. I really appreciate it.
  5. Feb 6, 2012 #4


    User Avatar
    Gold Member

    Re: Counter-weights

    Hopefully it works out and the thing is stable, as the centre of gravity is not being below the pivot points .
  6. Feb 7, 2012 #5
    Re: Counter-weights

    256- will a heavy base just sort this issue?

    If the lengths of rod were made from different materials and therefore had a different mass would this be an issue or is it just the distance between the weights that matters?

    just need to source a place to buy joints with as little friction as possible!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook