Question

A rocket fired vertically ascends with a constant acceleration g/3 for 1 min. its fuel is then all used and it continues to rise as a free body. What is the total time elapsed from the take off until the rocket strikes the
earth.
A) 60 s B) 2 min. C) 120 min. D) 20 min.

Answer 1

60seconds to the rocket

Answer 2

a = +g/3
t₁ = 60 sec
u = 0
v = ?

v = u+at
v = 0+ (g/3)(60)
v = 20g

v²-u² = 2ah
400g² = 2(g/3)h
h₁ = 600g
———————————
Now,
a = -g
u = 20g
v = 0
t₂ = ?

v = u+at
0 = (20g)+ (-g)t₂
t₂ = 20 sec

v²-u² = 2ah
-400g² = 2(-g)h
h₂ = 200g
____________________
Now,
u = 0
a = +g
H = h₁+h₂ = 800g
t₃ = ?

H = ut+ (1/2)at²
800g = (1/2)(g)t₃²
t₃ = √1600
t₃ = 40

=========================
Total time
= t₁+t₂+t₃
= 60+20+40
= 120 sec
= 2 min