(b) Difference in $KE = \frac{1}{2}m\left\lbrack \left( \sqrt{5gr} \right)^{2} - \sqrt{gr} \right\rbrack^{2}$ $= 2mgr = 2 \times 1 \times 10 \times 1 = 20\ $J
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.