Question

1. A dust packet is dropped from 9th
storey of a multi-storeyed building. In
the first second of its free fall another
dust packet is dropped from 7th storey
15 m below the 9th storey. If both pack-
ets reach the ground at same time, then
height of the building is​

Answer 1

Answer:

Explanation:

Lets take time taken by dust packet when dropped from 7th storey is t.

Then time taken by dust packet when dropped from 9th storey is t+1 s

From 7th storey

$h=\dfrac{1}{2}g\ t^2$

From 9th storey

$h+15=\dfrac{1}{2}g\ (t+1)^2$

From above two equations

$\dfrac{1}{2}g\ t^2+15=\dfrac{1}{2}g\ (t+1)^2$

5 t²+15 = 5 (t+1)²

t²+3 =  (t+1)²

t²+3 =  t²+1 +2 t

3 -1= 2 t

t = 1 s

$h=\dfrac{1}{2}g\ t^2$

h=5 x 1²

h= 5 m

H= 15 + 5 = 20 m

Height of building = 20 m