There are quite a few examples of this but I will go with what is common in physics which is "acceleration due to gravity".
Acceleration due to gravity is positive when an object is falling down in the direction of gravity but negative when it is thrown upwards and acting against gravity motion.
Thus;
g_downwards = 9.8 m/s²
g_upwards = -9.8 m/s²
If acceleration due to gravity is zero, it means that the object is at the centre of the earth because at that point g = 0.
