Forces are vectors, that is, they have a magnitude ("length", or "size") and a direction. As such, they cannot be described as either 'positive' or 'negative'. The latter are properties of scalars. read more
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. read more