The Spallation Neutron Source (SNS) project incorporates a superconducting radio-frequency (SRF) accelerator for the final section of the pulsed mode linac Cavities with geometrical β values of β =0.61 and β=0.81 are utilized in the SRF section, and are constructed out of thin-walled niobium with stiffener rings welded between the cells near the iris. The welded titanium helium vessel and tuner assembly restrains the cavity beam tubes Cavities with β values less than one have relatively steep and flat side-walls making the cavities susceptible to lsed RF induces cyclic Lorentz pressures that mechanically excite the cavities, producing a dynamic Lorentz force detuning different from a continuous RF system. The amplitude of the dynamic detuning for a given cavity design is a function of the mechanical damping, stiffness of the tuner/helium vessel assembly, RF pulse profile, and the RF pulse rate. This paper presents analysis and testing results to date, and indicates areas where more investigation is required. The medium β =.61 cavity is a thin wall (3.8mm) niobium structure that utilizes electron beam welding at the iris and equator (see Fig. 2). A stiffening ring is welded near the iris at an 80mm radius. The cavity is welded into a pure titanium helium vessel. One end of the cavity is welded directly to the helium vessel while the opposing end is attached to the helium vessel by the stainless steel tuner. The medium β cavities are susceptible to Lorentz force detuning because they have relatively large, flat sides that are flexible compared to very high β cavities.