Final answer:
The direction of acceleration during projectile motion without air resistance is always downward, toward the center of the Earth, due to gravity (9.81 m/s²). This acceleration occurs regardless of the object's movement being up or down.
Step-by-step explanation:
When an object is in projectile motion on level ground with no air resistance, the direction of the acceleration is downward, toward the center of Earth. This acceleration is due to gravity, commonly represented as g, which has a value of approximately 9.81 m/s² (or 32.2 ft/s²). An important aspect of projectile motion is that the horizontal and vertical motions are independent of each other. Since gravity only acts in the vertical direction, the only acceleration experienced by a projectile is in the vertical direction (the y-axis). Thus, the correct answer to the student's question is that the acceleration is downward. This is true even when the object is thrown upward or moving upward; the acceleration due to gravity still acts downward, continuously decreasing the object's upward velocity until it reaches zero at the peak of its trajectory, at which point it starts to fall back down.