Question

How long does it take for it to fall back to the earth? (in s)

• A. $24.6$
  
• B. $24$
  
• C. $25$
  
• D. $26$
  

Answer 1

Answer: (A)

$24.6$

Taking upward direction to be positive.
Initial velocity of missile $u=80m/s$
Displacement of missile $S=-1000m$
Using $S=ut+\frac{1}{2}g{t}^2$
where $g=-9.8m/s^2$
$\therefore -1000=80t-\frac{1}{2}\left(9.8\right)t^2$
$⟹9.8t^2-160t-2000=0$
$⟹t=24.6s$