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### Gravitation NCERT Solutions Class 11 Physics - Solved Exercise Question 8.17

Question 8.17:
A rocket is fired vertically with a speed of 5 km s–1 from the earth’s surface. How far from the earth does the rocket go before returning to the earth? Mass of the earth = 6.0 × 1024 kg; mean radius of the earth = 6.4 × 106 m; G= 6.67 × 10–11 N m2 kg2.
Solution:
Velocity of the rocket, v = 5 km/s = 5 × 103 m/s
Mass of the Earth, Me = 6 X 1024 kg
Radius of the Earth, Re = 6.4 X 106 m
Height reached by rocket mass, m = h
At the surface of the Earth,
Total energy of the rocket = Kinetic energy + Potential energy
= (1/2)mv2 + (-GMem / Re)
At highest point h,
v = 0
And, Potential energy = -GMem / (Re + h)
Total energy of the rocket = 0 + [ -GMem / (Re + h) ]
= -GMem / (Re + h)
From the law of conservation of energy, we have
Total energy of the rocket at the Earth’s surface = Total energy at height h
(1/2)mv2 + (-GMem / Re) = -GMem / (Re + h)
(1/2)v2 = GMe [ (1/Re) - 1 / (Re + h) ]
= GMe[ (Re + h - Re)  / Re(Re+ h) ]
(1/2)v2 = gReh / (Re + h)
Where g = GM / Re2 = 9.8 ms-2
∴ v2 (Re + h) = 2gReh
v2Re = h(2gRe - v2)
h = Rev2 / (2gRe -  v2)
= 6.4 X 106 X (5 X 103)2 / [ 2 X 9.8 X 6.4 X 106 - (5 X 103)2
h = 1.6 X 106 m
Height achieved by the rocket with respect to the centre of the Earth = Re + h
= 6.4 X 106 + 1.6 X 106  =  8 X 106 m