Acertain string just breaks when it is under400n of tension.
a boy uses this string to whirl a 10-kgstone in a horizontal circle
of radius 10m. the boycontinuously increases the speed of the
stone. atapproximately what speed will the string break?

The string will break with a speed of 20 m/s.

Explanation:

It is given that,

Tension at which the string just breaks, T = 400 N

Mass of the stone, m = 10 kg

Radius of the circle, r = 10 m

We need to find the speed at which the string will break. The boy continuously increases the speed of the  stone. The tension acting on the stone is equal to the centripetal force. It is a force that acts towards the center of circle. It is given by :

v = 20 m/s

So, the string will break with a speed of 20 m/s. Hence, this is the required solution.