Naked Science Forum

On the Lighter Side => Science Experiments => Topic started by: jules on 05/12/2017 12:10:26

Title: Balancing metre rulers
Post by: jules on 05/12/2017 12:10:26

I have been playing around with moving different masses each side of an unbalanced ruler to balance it.
I am not sure if the weight of the wooden ruler is part of the calculation or can be ignored. I am getting very strange results.
The clockwise moment about the pivot should equal the anticlockwise moment to get the ruler to balance.
With no masses added, the ruler balances at 50cm as expected.
The moment on each side is weight in N x distance in m. Units are Nm.
Example: An unbalanced ruler has a pivot 3/4 of the way along, at the 75cm (0.75m) mark. At the end of the longer arm is a mass of 100g (1N). I calculated that 300g (3N) is needed at the end of the shorter arm to balance this.
Actually doing it produces a different result, with over 450g needed on the shorter arm to balance the ruler.
I wonder if the weight of the wood in the ruler also needs to be taken into account, and how the calculation would look.

Title: Re: Balancing metre rulers
Post by: alancalverd on 05/12/2017 13:25:39

Yes indeed.

If the weight of the ruler is w, and you have lengths l and r cm on the left and right of the pivot, then you have an additional moment wl/200 on the left and wr/200 on the right.