A body of mass 1 kg tied to a string is just able to move in a vertical circular path of the radius 1 m . The difference between the kinetic energies at its highest and lowest positions of the body is

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Kinetic energy is the energy possessed by a body of mass m due to its velocity $v$ .

$K E = \frac{1}{2} m v^{2}$

At the highest point velocity, $v_{A} = \sqrt{r g}$

$∴ K E = \frac{1}{2} m v_{A}^{2}$

At the lowest point velocity, $v_{B} = \sqrt{5 r g}$

$K E = \frac{1}{2} m v_{B}^{2}$

Difference n $K E = \frac{1}{2} m (v_{B}^{2} - v_{A}^{2}) = \frac{1}{2} m (5 r g - r g)$

$= 2 m r g = 2 \times 1 \times 1 \times 10 = 20 J$

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