Answer
(a) No. The acceleration vector points straight down because of gravity. There is always a non-zero horizontal component of velocity, and so the velocity vector never points straight down. Therefore, the velocity vector and the acceleration vector are never parallel.
(b) Yes. When the projectile is at maximum height, the vertical component of velocity is zero. Then the velocity vector is parallel to the ground and perpendicular to the acceleration vector which points straight down toward the ground.
Work Step by Step
(a) No. The acceleration vector points straight down because of gravity. There is always a non-zero horizontal component of velocity, and so the velocity vector never points straight down. Therefore, the velocity vector and the acceleration vector are never parallel.
(b) Yes. When the projectile is at maximum height, the vertical component of velocity is zero. Then the velocity vector is parallel to the ground and perpendicular to the acceleration vector which points straight down toward the ground.
