Question

A rocket is fired with a speed u = 3from the earth surface. What will be its speed at interstellar space ?

• A. Zero
• B. $\sqrt { 2 \mathrm { gR } }$
• C. $\sqrt { 7 \mathrm { gR } }$
• D.

Answer 1

Answer: (C)

$\sqrt { 7 \mathrm { gR } }$

Answer 2

From conservation of mechanical energy

(K.E. + P.E.)_surface = (K.E. + P.E.)_infinity

$\frac { 1 } { 2 }$ m $\frac { 1 } { 2 }$m $\mathbf { v } _ { \infty } ^ { 2 }$ + 0

or $\sqrt { 7 \mathrm { gR } }$