Final answer:
An object projected straight upward is in one-dimensional projectile motion with gravity as the sole force acting upon it. At the highest point, its velocity becomes zero, and as it falls back, gravity causes it to accelerate downward consistently.
Step-by-step explanation:
An object projected straight upward into the air experiences projectile motion which is characterized by a path known as a trajectory. This specific case is a type of simple one-dimensional projectile motion where there is no horizontal displacement, and the only force acting on the object is gravity.
At the peak of its flight, the object's velocity is zero and begins to fall back to Earth, evidencing a change in velocity direction. Throughout the motion, both on the way up and down, the acceleration due to gravity remains constant and points downward, meaning the acceleration has the same sign in both directions.
All objects in projectile motion, whether they are subject to initial horizontal motion or not, fall at the same rate determined by their initial vertical velocity and the acceleration of gravity.