question: Why are things soft and hard? And why do hard things hurt more if you fall onto them or get hit with them?
comment: It stems from molecular bonds. Bonds have a certain length that they like to be at, so when they’re stretched, they try to restore themselves to that conformation. When you apply a force to it, you bend the molecule chain and stretch those bonds. Hard materials have stiff bonds, so their restoring force is much stronger than softer materials. As for the concrete/feather example, that has to do with momentum. In both cases, your momentum changes from 100% to 0%, but the pile of feathers is going to slow you down over the course of several fractions of a second because of how much more the bonds stretch, compared to near instantaneously on stiff concrete. The rate of change of your momentum is directly proportional to the force you feel.
passages: Section::::Practical effects.
  
  Surface structure plays a key role in governing contact mechanics, that is to say the mechanical behavior exhibited at an interface between two solid objects as they approach each other and transition from conditions of non-contact to full contact. In particular, normal contact stiffness is governed predominantly by asperity structures (roughness, surface slope and fractality) and material properties.
   In European martial arts the same scale applies, especially in the German style of grappling and swordplay dating from the 14th century ("e.g.", the German school of fencing); the use of the terms hard and soft are otherwise translated as "strong" and "weak." In later European martial arts the scale becomes less of a philosophic concept and more of a scientific approach to where two swords connect upon one another and the options applicable to each in the circumstance, parting further from the original meaning as time went along.
   In Imperial units, stiffness is typically measured in pounds(lbs) per inch.
   The amount of dilation depends strongly on the density of the soil. In general, the denser the soil the greater the amount of volume expansion under shear. It has also been observed that the angle of internal friction decreases as the effective normal stress is decreased.
   Further advancement in the field of contact mechanics in the mid-twentieth century may be attributed to names such as Bowden and Tabor. Bowden and Tabor were the first to emphasize the importance of surface roughness for bodies in contact. Through investigation of the surface roughness, the true contact area between friction partners is found to be less than the apparent contact area. Such understanding also drastically changed the direction of undertakings in tribology. The works of Bowden and Tabor yielded several theories in contact mechanics of rough surfaces.
   Because of dilatancy, the angle of friction increases as the confinement increases until it reaches a peak value. After the peak strength of the soil is mobilized the angle of friction abruptly decreases. As a result, geotechnical engineering of slopes, footings, tunnels, and piles in such soils have to consider the potential decrease in strength after the soil strength reaches this peak value.
   The phenomenon of dilatancy can be observed in a simple shear test on a sample of dense sand. In the initial stage of deformation, the volumetric strain decreases as the shear strain increases. But as the stress approaches its peak value, the volumetric strain starts to increase. After some more shear, the soil sample has a larger volume than when the test was started.
   Hardness of a material to deformation is dependent on its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as its bulk modulus or Young's modulus. Stiffness is often confused for hardness. Some materials are stiffer than diamond (e.g. osmium) but are not harder, and are prone to spalling and flaking in squamose or acicular habits.
  
  Section::::Physics.:Mechanisms and theory.
   The modern distinction between hard and soft science is often attributed to a 1964 article published in "Science" by John R. Platt. He explored why he considered some scientific fields to be more productive than others, though he did not actually use the terms themselves. In 1967, sociologist of science Norman W. Storer specifically distinguished between the natural sciences as hard and the social sciences as soft. He defined hardness in terms of the degree to which a field uses mathematics and described a trend of scientific fields increasing in hardness over time, identifying features of increased hardness as including better integration and organization of knowledge, an improved ability to detect errors, and an increase in the difficulty of learning the subject.
   Dilatancy (granular material)
  
  Dilatancy is the volume change observed in granular materials when they are subjected to shear deformations. This effect was first described scientifically by Osborne Reynolds in 1885/1886 and is also known as Reynolds dilatancy.
   When two macroscopically smooth surfaces come into contact, initially they only touch at a few of these asperity points. These cover only a very small portion of the surface area. Friction and wear originate at these points, and thus understanding their behavior becomes important when studying materials in contact. When the surfaces are subjected to a compressive load, the asperities deform through elastic and plastic modes, increasing the contact area between the two surfaces until the contact area is sufficient to support the load.
   In terms of psychophysics, several studies have been done involving a person’s ability to detect the differences between the weight and roughness of objects. A syndrome called Verger-Dejerine syndrome has been known to affect these somatosensory abilities. Patients with this somatosensory cortical loss syndrome commonly display damage to their parietal lobe and it was eventually concluded that it may be that the brain has some form of an asymmetrical organization, as performance in a normal subject shows oblique differences depending on their hand use. However, these patients still exhibit normal or minimally reduced peripheral sensitivity to cold, heat, pain, touch and deep pressure.
   May be cut smoothly with a knife. Relatively few minerals are sectile. Sectility is a form of tenacity and can be used to distinguish minerals of similar appearance. Gold, for example, is sectile but pyrite ("fool's gold") is not.
  
  Elasticity:br
  
  If bent, will spring back to its original position when the stress is released.
  
  Plasticity:br
  
  If bent, will not spring back to its original position when the stress is released. It stays bent. In contrast, flexibility is the ability of a material to deform elastically and return to its original shape when the applied stress is removed.
   BULLET::::- "Deflection" is a term to describe the magnitude to which a structural element is displaced when subject to an applied load.
  
  Section::::Definition.:Stress–strain relations.
  
  BULLET::::- "Elasticity" is the ability of a material to return to its previous shape after stress is released. In many materials, the relation between applied stress is directly proportional to the resulting strain (up to a certain limit), and a graph representing those two quantities is a straight line.
   Impact resistance decreases with an increase in the modulus of elasticity, which means that stiffer materials will have less impact resistance. Resilient materials will have better impact resistance.
   When hitting a textured surface at an angle, hard light will accentuate the textures and details in an object.
  
  Section::::Fall-off.
  
  Light intensity tends to dim with distance. For a point source of light, intensity decreases as distance increases. Intensity ("I") is inversely proportional to the square of the distance ("D"), as expressed in the formula .
   The contact between surfaces, in reality, is a contact between roughness and the origin of the phenomenon of friction, and therefore of the dissipation of energy, is due precisely to the deformations that such bumps undergo due to the load and of relative movement. Plastic, elastic or rupture deformations can be observed:
  
  BULLET::::- Plastic Deformations – permanent deformations of the shape of the bumps;
  
  BULLET::::- Elastic Deformations – deformations in which the energy expended in the compression phase is almost entirely recovered in the decompression phase (elastic hysteresis);
   BULLET::::- "Tensile stress" is the stress state caused by an applied load that tends to elongate the material along the axis of the applied load, in other words the stress caused by "pulling" the material. The strength of structures of equal cross sectional area loaded in tension is independent of shape of the cross section. Materials loaded in tension are susceptible to stress concentrations such as material defects or abrupt changes in geometry. However, materials exhibiting ductile behavior (most metals for example) can tolerate some defects while brittle materials (such as ceramics) can fail well below their ultimate material strength.
   BULLET::::- The frequency of citations of journal articles, considered in the network of all citations amongst all papers, as a function of the number of citations in a given paper.
  
  BULLET::::- The formation and propagation of cracks and tears in materials ranging from steel to rock to paper. The variations of the direction of the tear, or the roughness of a fractured surface, are in power-law proportion to the size scale.
  
  BULLET::::- The electrical breakdown of dielectrics, which resemble cracks and tears.
   Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces. Roughness is often a good predictor of the performance of a mechanical component, since irregularities on the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion. Generally speaking, rather than scale specific descriptors, cross-scale descriptors such as surface fractality provide more meaningful predictions of mechanical interactions at surfaces including contact stiffness and static friction.
   BULLET::::- "Plasticity" or plastic deformation is the opposite of elastic deformation and is defined as unrecoverable strain. Plastic deformation is retained after the release of the applied stress. Most materials in the linear-elastic category are usually capable of plastic deformation. Brittle materials, like ceramics, do not experience any plastic deformation and will fracture under relatively low strain, while ductile materials such as metallics, lead, or polymers will plasticly deform much more before a fracture initiation.
   Section::::Definition.:Strain (deformation) terms.
  
  BULLET::::- "Deformation" of the material is the change in geometry created when stress is applied ( as a result of applied forces, gravitational fields, accelerations, thermal expansion, etc.). Deformation is expressed by the displacement field of the material.
   Another function of bones is the storage of certain minerals. Calcium and phosphorus are among the main minerals being stored. The importance of this storage "device" helps to regulate mineral balance in the bloodstream. When the fluctuation of minerals is high, these minerals are stored in bone; when it is low it will be withdrawn from the bone.
  
  Section::::The different muscles.:Muscular.
   Hardness
  
  Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials (e.g. metals) are harder than others (e.g. plastics, wood). Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: "scratch hardness", "indentation hardness", and "rebound hardness".
  
  Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity.
  
  Common examples of hard matter are ceramics, concrete, certain metals, and superhard materials, which can be contrasted with soft matter.
  
  Section::::Measuring hardness.
   BULLET::::- "Shear stress" is the stress state caused by the combined energy of a pair of opposing forces acting along parallel lines of action through the material, in other words the stress caused by faces of the material "sliding" relative to one another. An example is cutting paper with scissors or stresses due to torsional loading.
  
  Section::::Definition.:Strength terms.
  
  Mechanical properties of materials include the yield strength, tensile strength, fatigue strength, crack resistance, and other characteristics.
  