Acceleration due to gravity is generally considered to be negative because the force of gravity pulls in the downward direction. The frame of reference from Earth's surface most often used in physics has the sky in the upward direction and the ground in the downward direction.
The acceleration due to gravity on Earth rounded to the nearest hundredth is -9.81 meters per second squared. This means that if no forces other than that of gravity are present, then for every second an object is falling, its velocity towards Earth increases by 9.81 meters per second. Negative acceleration does not necessarily mean that the object is slowing down. Speed is the magnitude of how fast an object is moving, while velocity is the magnitude and the direction of the movement. If the acceleration and velocity are in the same direction, then the speed is increasing, meaning that the object is speeding up. If acceleration is in the opposite direction of velocity, then the speed is decreasing and the object is slowing down. In any frame of reference in physics, mathematics or other uses, down and left are generally considered the negative directions, while up and right are generally considered to be positive. Frames of reference are most often used in physics; they keep positive and negative directions universal during the analysis of a problem.