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## Homework Statement

Find the number of kilograms needed to pull apart evacuated Magdeburg Hemispheres whose radii are 25.0 cm when the barometer reads 760mm of Hg.

I really don't know where to start. Could someone walk me through it?

## Homework Equations

Unknown.

## The Attempt at a Solution

I read somewhere that an equation to help is F=pi*r^2, so I made 25.0cm into 0.250m, and made F=pi*0.0625... but that doesn't seem right.