A person sitting on the ground floor of a building notices through the window, of height 1.5 m, a ball dropped from the roof of the building and crosses the window in 0.1 s. What is the velocity of the ball when it is at the topmost point of the window? (g-10 m/$s^2$)

(1) 20 m/s

(2) 15.5 m/s

(3) 14.5 m/s

(4) 4.5 m/s

Step 1, Given data and figure

The above situation can be represented as

Now for the journey from A to B

(Upward direction is taken as positive direction)

Distance, S = -h = -1.5 m

Acceleration = -g = 10 m/$s^2$

Initial velocity = -v

Time t = 0.1 sec.

Step 2, Finding the velocity

From the equation of motion

We know,

$s=ut+\frac{1}{2}a{t}^2$

Putting all the values

$-1.5=(-v)\times 0.1+\frac{1}{2}\times (-10)\times {0.1}^2$

After solving

v = 14.5 m/s

Hence the speed of the ball when it is at the topmost point of the window is 14.5 m/s