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Farrell & Greacen (1966) and Greacen et al. (1968) calculated the pressure required to expand cavities in the soil by spherical and cylindrical deformation respectively. The advancing probe or root was accommodated by compression of the surrounding soil. This was assumed to occur in two distinct regions: an inner zone of compression with plastic failure immediately surrounding the probe, and a zone of elastic compression outside this. Sharp penetrometers (5" semi-angle) and plant roots were assumed to deform the soil cylindrically, whereas blunt penetrometers caused spherical deformation. In calculations for three. | Lecture 6 Quantum mechanical spin Background o Until now we have focused on quantum mechanics of particles which are featureless - carrying no internal degrees of freedom. A relativistic formulation of quantum mechanics due to Dirac and covered later in course reveals that quantum particles can exhibit an intrinsic angular momentum component known as spin. However the discovery of quantum mechanical spin predates its theoretical understanding and appeared as a result of an ingeneous experiment due to Stern and Gerlach. Spin outline rộ Stern-Gerlach and the discovery of spin 2 Spinors spin operators and Pauli matrices ft Spin precession in a magnetic field ft Paramagnetic resonance and .