Question

A stone is dropped from the roof of a building; 1.50 s after that, a second stone is thrown straight down with an initial speed of 21.0 m/s , and the two stones land at the same time. A) How long did it take the first stone to reach the ground? b) How high is the building? c) What are the speeds of the two stones just before they hit the ground? Enter your answers numerically separated by a comma.

Answer 1

Given is:-

Time difference between stones = 1.50 seconds

The initial speed of the second stone is u=21 m/s

Now,

part-a

Using the second equation of motion we get

for first stone

eq-1

for second stone

eq-2

but we know that  

by plugging this value in eq-2 we get

eq-3

by equating eq-1 and eq-3 we get

which gives us

Hence,

time taken by first stone to reach the ground is 3.25 seconds

Part-b

By plugging the value of t₁ in eq-1 we get

which gives us

Part-c

Using first equation of motion we get

thus

similarly

which gives us