Physics stuff, plz clear my doubt! ?

Question

A rifle is aimed horizontally at a target 100m away. The bullet hits the target 0.15m below the aiming point.

a) What's the bullet's time of flight?

b) What is the muzzle velocity of the rifle?

Answer 1

Let's say that the acceleration due to gravity is 9.8 m/s².  Distance covered from a stand still (zero vertical veleocity) is ½at².

0.15m=½ x 9.8 m/s² x t².  Solve for t, t=.01237 seconds.

Assuming this all took place in a vacuum, so that we can neglect horizontal deceleration of the bullet, muzzle velocity = distance/0.1237 seconds = 808.3 m/sec.

Answer 2

a) Since Voy = 0 then

y(t) = -1/2*g*t^2

=> t^2 = -2y(t)/g

t^2 = (-2 * -.15m)/(9.8m/s^2)

t ~ .175 s

b) vox = Δx/t = 100m/.175s ~ 571.43 m/s