Question

If a stone is thrown with a speed $v_0$ at an angle ${\theta }_0$ with horizontal, find the velocity of the stone when its line of motion makes an angle $\theta$ with horizontal.

Answer 1

The horizontal component of the velocity will be constant during the projectile motion. Horizontal component of the initial velocity, $u_x=v_{0}cos{\theta }_0$. Horizontal component of the velocity when the particle makes an angle $\theta$ with horizontal, $v_x=vcos\theta$.
Hence, $vcos\theta =v_{0}cos{\theta }_0\Rightarrow v=\frac{v_{0}cos{\theta }_0}{cos\theta }$