Centre of Mass

Discussion in 'Physics' started by edwardholmes91, Mar 15, 2013.

  1. edwardholmes91

    Thread Starter Active Member

    Feb 25, 2013

    I am designing a clock at the moment and the idea for the casing is to sandwich two PCBs stacked on top of each other between a flat piece of perspex and also a bent piece at an angle.

    Code ( (Unknown Language)):
    1.                     /--[COLOR="DarkGreen"]/--/[/COLOR]-/
    2.                    /  [COLOR="DarkGreen"]/  /[/COLOR] /
    3.                   /  [COLOR="DarkGreen"]/  /[/COLOR] /
    4.                  /--[COLOR="DarkGreen"]/--/[/COLOR]-/
    5.                 /     [COLOR="DarkGreen"]/[/COLOR] /
    6.                /    [COLOR="DarkGreen"] /[/COLOR] /
    7.               /     [COLOR="DarkGreen"]/[/COLOR] /
    8.              /-----[COLOR="DarkGreen"]/[/COLOR]-/_______
    I have attempted to draw my idea using characters above. The green is for PCBs and the black is 3mm perspex.

    So my question is, how to I know what length to make the base? And also what angle it needs to be at?

    Currently the longest "upright" section is about 6 inches and the base is about 3" and the upright is at 70% from the base.

    I haven't got the parts made yet, so I don't have any way of weighing them, but they shouldn't be too heavy. What I don't want is to manufacture the parts to find it either falls forward and won't stand up or it topples back.

    I will be laser cutting the perspex to get a nice clean cut and smooth finished surface, then using a strip heater to make the bend. Ideally I don't want to have to heat the bend more than once, because it will look messy. Once an angle has been established, I can make a simple bending jig to fold the plastic around.

    Any guestimations or ideas/theory would be most appreciated.
    Last edited: Mar 15, 2013
  2. #12


    Nov 30, 2010
    1) What is perspex?
    2) A GOOD scale drawing would practically answer the question by itself.
  3. WBahn


    Mar 31, 2012
    I don't know what you mean when you say that the upright is 70% from the base.

    The location of your CG for the upright portions will depend on the components, particularly the display, that is mounted on the PCBs.

    One approach might be to make a worst case estimate for where the CG might end up. This will give you a box (rectangular area). Then decide how much of an angle you want for aesthetic purposes. Plot you box at that angle and drop a "shadow" straight down. In theory, as long as your base includes the shadow, the thing should be stable. But you might not have much margin, so extend it out in both directions by some fraction of the length of the shadow, say 25% or 50%.

    If, instead, you are constrained with fixed upright and base lengths, the choose the angle that pusts your best guess at the CG of the upright portion over the midpoint of the base. Then check to see if your worst case box shadow is over the base. If it extends off one end of the base, see how much your best guess CG would move if you changed the angle to get your worst case box shadow onto the base and ask yourself if that is reasonable (i.e., you haven't moved the best guess CG too far from the center).
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    I think he means "70 degrees" to the base, not 70%?

    If the front box section was symmetrical the CG is right in the centre of the box. In your diagram that CG would be forward from the front edge of the base so it would fall forward UNLESS your base was heavy enough to counter that.

    For general design I would keep the front part of the box light (thin perspex) and the back part of the box heavier, to move the CG back. Then make the base heavy and if needed put a weight bar like a shiny metal rod along the rear of the base.

    Or, just bring the front edge of the base forward enought to be below the centre of the box. That will work fine but might compromise the visual effect you are aiming for.
  5. Metalmann

    Active Member

    Dec 8, 2012
    Since you don't know the exact weight, it's just guesswork about the angles.

    So, I'd guess you are in the ballpark as it is.;)
  6. edwardholmes91

    Thread Starter Active Member

    Feb 25, 2013
    Sorry to clarify... I did mean 70 degrees. The rough picture isn't accurate because the boards and pieces of perspex are all lined up... in effect making a 6" by 4" by 1" ish box which will be at 70 ish degrees to the base. So as long as the exact centre of the box is roughly over the base which I guessed is roughly 3" in my original sketch then it should be ok?
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Perspex is a brand name for what the US calls Plexiglas or Lucite. An acrylic plastic. Think Perspex is from UK.
    #12 likes this.
  8. WBahn


    Mar 31, 2012
    It definitely needs to be over the base -- no roughly about that. You want it to be roughly centered over the base.