41.00.00 Electromagnetism; electron and ion optics
42.00.00 Optics (for optical properties of gases, see 51.70.+f; for optical properties of bulk materials and thin films, see 78.20.-e; for x-ray optics, see 41.50.+h)
43.00.00 Acoustics (for more detailed headings, see Appendix to section 43)
44.00.00 Heat transfer
45.00.00 Classical mechanics of discrete systems
46.00.00 Continuum mechanics of solids (see also 83.10.Ff in rheology; 91.60.Ba Elasticity, fracture, and flow; 91.45.Ga Dynamics and mechanics of tectonics; 91.55.Ln Kinematics of crustal and mantle deformation in geophysics)