舊課程就中六讀既
未知新高中有冇得讀喇
所以唔敢寫F4..又唔敢寫F7

題目是咁的:


Three identical bricks A , B and C are placed on the edge of a table as shown. Each brick is of length 24.0cm and C overhangs the table by x cm. What is the maximum value of x so that the system can still be in equilibrium ?

先謝解答

As C overturns, B will be tipped slightly.

First take moment at the bottom-left corner of B.
_CF_B(18) - mg_B(12) = 0
_CF_B = _BF_C = (2/3)(mg_B) (∵Action-reaction pair)

Then take moment at the edge of the table.
_BF_C(24-x) - mg_C(x-12) = 0
(2/3)(mg_B)(24-x) = mg_C(x-12)
∵mg_B = mg_C
∴x_max = 16.8