一條Physic,,求full solution (10分)

ANS: A

consider vertical direction
as the block is placed on a table,
there is a reaction force R acting on the block so that the block is stationary
unless the up lifting force is greater than its weight
as 100N > 60N, net vertical force = 0N

consider horizontal direction
there is only one force of magnitude 30N towards the left acting on the block
therefore, net horizontal force = 30N

resultant force = sqrt((vertical force)^2 + (horizontal force)^2) = 30N

2011-06-08 22:57:34 補充：
the answer of 天同 did not consider the reaction force by the table

A.The above explanation is enough

The forces acting on the block are:
Net horizontal force = 30 N
Net downward vertical force = (100 - 60) N = 40 N

Hence, resultant force = square-root[30^2 + 40^2] N = 50 N
The answer is option C