A spring is stretched by 3 cm, its potential energy is `U_1`. If the spring is stretched by 6 cm, its potential energy be that is stored in it is `U_(2)="……"U_(1)`. (Fill in the blank).

Explanation

`U_(1) = (1)/(2)ky_(1)^(2)= (1)/(2)k(3)^(2)= (1)/(2)kxx 9`
`U_(2)= (1)/(2)ky_(2)^(2)= (1)/(2)k(6)^(2)= (1)/(2)kxx 36`
`therefore (U_2)/(U_1)= (36)/(9)= 4`
`therefore U_(2) = 4U_(1)`.