A stone is thrown upwards from the roof with a velocity 15ms^-1 at an angle 30degree to the horizontal. The height of the building is 40m.calculate the magnitude of the velocity of the stones before it strikes the ground

Answer 1

Vi up = 15 sin 30 = 7.5 m/s initially
u = 15 cos 30 = 13 m/s forever
Hi = 40
then
h = Hi + Vi t - 4.9 t^2
0 = 40 + 7.5 t - 4.9 t^2
or 4.9 t^2 - 7.5 t - 40 = 0
t = [ 7.5 +/- sqrt(56.25 + 784) ] / 9.8
use + root of course
t = 3.72 seconds
then
v = Vi - 9.8 t = 7.5-36.5 = -29 m/s down
so
|v| = sqrt( 169 + 841) = 31.8 m/s