A bullet is fired vertically up from a 400 m tall tower with a speed

• GP Motion Force

A bullet is fired vertically up from a 400 m tall tower with a speed 80 m/s. If g is taken as 10 m/s², the time taken by the bullet to reach the ground will be

1. 24 s
2. 20 s
3. 16 s
4. 8 s

Solution

When bullet goes up,

0 = 80 - 10t (Use equation: v = u + at)

t = 8 s

s = (80 x 80) / (2 x 10) (Use equation: v² - u² = 2as)

s = 320 m

When bullet goes down,

s = ut + ½at²

400 + 320 = ½ x 10 x t²

t = 12 s

So, total time = 8 + 12 = 20 s

The correct option is B.