why stress & pressure are two different physical quantities though they have same formula force/area?

Answer 1

What is called stress and pressure both describe Energy per unit volume.
Wether you observe pressure or stress of space is all dependent on the observer's frame of reference.
If you view energy in a longitudinal direction of a particular mass volume you see it as stress. If you view the Gravity energy of the Earth as Energy in thee dimensional volume ,then you see it as the Pressure of Gravity.
The law of Gravitation force that Newton indicated in his equation would be an example of stress between two masses.Einstein described it in terms of a distortion of space as a function of its curvature. The Einstein Equation is : Guv= K Tuv
Guv is the Curvature Tensor of Space and Tuv is the Stress Tensor of Space. & K is a constant of proportionality.