Question

A rock is dropped from the top of a tall building. The rock's displacement in the last second before it hits the ground is 40 %% of the entire distance it falls. How tall is the building?

Answer 1

Let, Height of building = H

Let, Time taken by rock to reach the ground = T

  Given, initial velocity of rock=0

Dispalcement of rock in last second =

We know that displacement covered in last second is given by     

also , ----------------1)

So,   -------------2)

Dividing 1) and 2)

  

so, T=4.40s

Putting this value of T in equation 1),we get hieght of building as