应力

The definition of stress in physics is "force per unit area".

Enough stress on an object can cause strain or deformation.

The notion of stress can be applied to our advantage in the real world. Consider using a knife (to increase stress) and using snowshoes (to decrease stress).

Suppose we want to cut an apple with a knife. If we start with a dull knife, we cannot break the skin. But as the dull blade is sharpened to a finer and finer edge, the contact area is decreased. So for the same amount of force as with the dull knife, the sharp blade produces more stress than the dull blade and the higher stress causes deformation (cutting). A sharp blade increased our stress on the apple.

Consider snowshoes: they increase the area that our constant weight (force) is distributed over. So without snowshoes, our (constant) weight (force) generates too much stress and we break through the snow surface. With snowshoes, area is bigger for the same amount of force (weight), so stress is reduced and no snow deformation results i.e. we walk on top of the snow without breaking the surface. Snowshoes reduced our stress on the snow.

Stress is more like pressure. Pressure is the force per unit area.

Stress is the same except that pressure acts on a surface and stress acts inside the object. For example when you exert force on a top of a fixed pole or stick the pressure on the surface of pole is force/area of contact. Pole exerts this force on another object (ground etc) through its other end. So there is a force exerted on the molecules which is in middle of the pole by the one above it. The force on every cross section is equal to the force you exerted. The force on the middle section per unit cross section is stress.