The distance, d, through which a stone falls from rest varies directly as the square of the time, t, taken. If the stone falls 45cm in 3 seconds, how far will it fall in 6 seconds?

A.

90cm

B.

135cm

C.

180cm

D.

225cm

Correct answer is C

d \(\alpha\) t2

d = t2 k

where k is a constant. d = 45cm, when t = 3s; thus 45 = 32 x t

k = \(\frac{45}{9}\) = 5

thus equation connecting d and t is d = 5t2

when t = 6s, d = 5 x 62

= 5 x 36

= 180cm