Question

A body is projected such that it's kinetic energy at the top is 0.75 of initial kinetic energy . what is the initial angle of the projected of the projectile with the horizontal.

Answer 1

A good question .
Well ...
It's given that the kinetic energy of the body at top point of projectile is 0.75 times of its initial kinetic energy.
We know that velocity at top will be v cos x .
So atq
1/2 m v^2cos^2x = (0.75) 1/2mv^2
Solving this we will eliminate 1/2mv^2 from both the side . We will be left with cos^2x = 0.75 => cos^2x = 75/100
This gives cos^2x =3/4 . Therefore cos x = √3/4
X = 30°