Question

A stone is tied to a string of length 'l' and is whirled in a vertical circle with the other end of the string as the center. At a certain instant of time, the stone is at its lowest position and has a speed of 'u'. The magnitude of the change in velocity as it reaches a position where the string is horizontal (g being acceleration due to gravity) is

• A. $\sqrt{u^2-gl}$
• B. $u-\sqrt{u^2-2gl}$
• C. $\sqrt{2gl}$
• D. $\sqrt{2(u^2-gl)}$

Answer 1

Answer: (D)

$\sqrt{2(u^2-gl)}$

Answer 2

The magnitude of the change in velocity as a stone reaches a position where the string is horizontal, starting from its lowest point with speed 'u', is 2u.