r त्रिज्या के ऊध्र्वाधर वृत्त में, मार्ग क

  Let the tension in the string at the highest point be $T$ Minimum speed required by the particle at the highest point to complete the vertical circular motion is $\sqrt{g r}$

$\therefore \quad \frac{m v^{2}}{r}=T+m g$

OR $\quad \frac{m(g r)}{r}=T+m g \quad \Longrightarrow T=0$

Thus tension can be zero at the highest point.