✔ 最佳答案
By the formula v^2= u^2 +2as,
v^2 = 2as (Since the initial speed of the rock is zero)
v= sqrt( 2as ) (sqrt =square root )
Let the height of the second floor = d , the seventh floor = 3.5d
Consider when the rock is released from the second floor:
v = sqrt( 2as )
v = sqrt( 2ad ) ------ *
Consider when the rock is released from the seventh floor:
v" = sqrt( 2as )
v" = sqrt( 2a(3.5d) )
v" = sqrt(3.5) * sqrt( 2ad)
Therefore, v" = sqrt (3.5) v (by * )
v" = 1.87 v
The answer is B
Please correct me if I have made any mistakes, or there is a faster way to solve the question ^_^ !