## Physics (10th Edition)

Since the only force acting on the projectile is in the downward direction and is due to gravity, the acceleration is always in the downward direction. Since the vertical component of the velocity is zero at the topmost point, the velocity becomes horizontal at that point. Since the velocity at that point is horizontal and the acceleration is vertical, they are perpendicular. The velocity and acceleration can never become parallel unless the angle $\theta$ is not equal to 90 degree