Solids

Everything in the universe is made of atoms, yet the same substance can exist as a solid, a liquid, or a gas -- water as ice, liquid water, or steam is the standard example, even though all three forms are built from the same water molecules. What decides which of the three states a substance is in, at a given moment, is the distance between its atoms or molecules and the strength of the bonding between them. In a SOLID, atoms or molecules are tightly bound by interatomic bonding and sit fixed at particular mean positions, vibrating about those positions but not wandering away from them. When a solid is heated, its atoms absorb thermal energy and vibrate more vigorously about their mean positions; once the temperature crosses the melting point, this thermal energy is enough to break the interatomic bonding, and the atoms gain enough energy to wander around -- this is the LIQUID state, where intermolecular forces are still important but the structure has become mobile rather than rigid. Heating a liquid further at constant pressure to its boiling point (or lowering the pressure on it at constant temperature) turns it into a GAS by evaporation; gas molecules have either extremely weak bonds or none at all, so they move freely and rapidly, which is why a gas both takes the shape of its container and expands to fill it completely. (Physics also recognises other, more extreme states of matter beyond these three -- plasma and Bose-Einstein condensates among them -- which arise only in extreme environments; a large part of the visible matter in the universe is in fact hot plasma, found in stars and in the rarefied interstellar medium.) In ordinary Newtonian mechanics, real objects are often idealised as point masses or as perfectly rigid bodies whose shape never changes under an applied force. Real materials are not perfectly rigid: when a force acts on them, they deform, if only slightly. Section 7.2 develops the physics of exa …