Vertically downwards.
Explanation
Acceleration due to gravity ($g$) is the acceleration imparted to an object solely due to the gravitational force of the Earth. As a vector quantity, it possesses both magnitude and direction. The fundamental property of Earth's gravity is that it is an attractive force directed towards the center of the Earth.
Detailed Analysis
The direction of acceleration is determined by the direction of the net force acting on the object. In the case of a projectile moving under the influence of gravity alone (ignoring air resistance):
- The gravitational force exerted by the Earth acts vertically downwards towards the Earth's center at all times.
- Consequently, the acceleration due to gravity is always directed vertically downwards.
- This direction remains constant regardless of the object's motion. During the ascent, the object's velocity is directed upwards, while the acceleration is downwards. This opposition between velocity and acceleration causes the object to decelerate as it rises.
Key Takeaway
Acceleration due to gravity is always directed vertically downwards, irrespective of whether the object is moving upwards, downwards, or is momentarily at rest at the highest point.