A boy climbs to the roof of his house and drops a 0.40 kg rubber bll from the 7.5-meter peak of the house. how fast will the ball be travelling when it hits the ground?
a. 15 m/s
b. 24 m/s
c. 30 m/s
d. 12 m/s
Well if you say g = 10 m/s^2 approximately on earth then
h = (1/2) g t^2
t = sqrt (2h/g)
then
v = -g t = -g sqrt (2h/g) = -sqrt(2g h)
OR
use conservation of energy
m g h = (1/2) m v^2
v^2 = 2 gh
v = sqrt (2 g h) (it does not give you the direction of course but same speed)
= sqrt (2*10*7.5) = sqrt (2*3*25) =5 sqrt 6 = about 12