KinematicsHard
Question
An object is thrown horizontally from a point ′A′ from a tower and hits the ground 3s later at B. The line from ′A′ to ′B′ makes an angle of 30o with the horizontal. The initial velocity of the object is : (take g = 10 m/s2)
Options
A.15√3 m/s
B.15 m/s
C.15√3 m/s
D.25/√3 m/s
Solution
AC = 
gt2 = 45 m BC = 45√3 m = u.t u = 
= 15 √3 m/s.
Object is thrown horizontally so ux = v & uy = 0
from Diagram
− y = uy t −
g t2
y =
×10 × (3)2
y = 45m ........(2)
& tan 30o =
⇒ y √3 = x ............(2)
& x = v t = 3v .................(3)
from equation (1), (2) & (3)
45√3 = 3v
v = 15 √3 m/s
gt2 = 45 m BC = 45√3 m = u.t u = = 15 √3 m/s.Object is thrown horizontally so ux = v & uy = 0
from Diagram
− y = uy t −
g t2y =
×10 × (3)2y = 45m ........(2)
& tan 30o =
⇒ y √3 = x ............(2) & x = v t = 3v .................(3)
from equation (1), (2) & (3)
45√3 = 3v
v = 15 √3 m/s
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