Question

A stone of mass 10kg is suspended vertically with breaking stremgth 30kgf minimum time in which stone cam be raised through a height 10m starting from rest ans.1s

Answer 1

Mass of stone = 10kg
breaking strength = 30kg wt = 30×g = 30g N (g=acceleration due to gravity)

let the maximum acceleration allowed = a
So breaking strength = mg+ma
⇒ 30g = 10g + 10a
⇒ 30g - 10g = 10a
⇒ 10a = 20g
⇒ a = 20g/10 = 2g
⇒ a = 2×10 = 20m/s²

so maximum acceleration allowed = 20m/s²
h = 10m
u = 0
let the time taken = t

h = ut + 0.5at²
⇒ 10 = 0 + 0.5×20×t²
⇒ 10= 10t²
⇒ t² = 10/10 = 1
⇒ t = √1= 1s

So minimum time is 1s.