A stone of mass $1\;kg$ is tied to the end of a string of $1\;m$ long. It is whirled in a vertical circle. If the velocity of the stone at the top be $4\;m/s$. What is the tension in the string?

Given : $v =4$ $m/s$ $m =1 kg$ $r = 1$ m

Using circular motion equation : $\dfrac{mv^2}{r} =T + mg$

$\implies$ $T =\dfrac{mv^2}{r} -mg$

$\therefore$ $T =\dfrac{1 \times 4^2}{1} -1 \times 10$ $\implies T = 6$ $N$