# Thread: Mechanics problem centre of mass

1. ## Mechanics problem centre of mass

i cant figure out as to how to solve part (ii) i know that i should take moments about O i also know that we should get 0.36 for the distance of centre of gravity from o for that hole but why are we taking the area of that thing that isnt there

2. ## Re: Mechanics problem centre of mass

solved it just now we i dont get the last part which is

"It is possible to increase the length of the cylindrical hole so that the solid still has its centre of
mass at O. State the increase in the length of the hole."
answer is 0.24m examiner did Increase in length [= 2 × (0.6 – 0.48)] = 0.24m i dont get it help

3. ## Re: Mechanics problem centre of mass

The "trick" here is to realize that initially the hole leaves 0.12 of material between the bottom of the hole and O, and that if you dig 2 x .12 deeper the removed material is "balanced" about point O; hence removing that extra amount doesn't affect the center of mass of the assembly. Stated another way - if for every portion of material you remove at distance x to the right of O you also remove material that same distance x to the left of O, then the center of gravity is unchanged.