Question

A mass m is raised from a distance 2R from surface of earth to 3R. Work done to do so against gravity will –

• A. $\frac{mgR}{10}$
• B. $\frac{mgR}{11}$
• C. $\frac{mgR}{12}$
• D. $\frac{mgR}{14}$

Answer 1

Answer: (C)

$\frac{mgR}{12}$

Answer 2

U_i = – $\frac{GMm}{(2R + R)}$

U_f= – $\frac{GMm}{(R + 3R)}$

W = DU = U_f – U_f

= $\frac{GMm}{12R}$ = $\frac{GM}{R^{2}}$ × $\frac{mR}{12}$

= mgR/12