Abstract: A simply supported beam struck by a pendulum is analyzed under the imperfect elasticity impact condition. By analyzing the pendulum potential energy differences before and after the impact, a relatively simple formula has been derived based on the energy theorem, which can satisfactorily give the value of the dynamic load coefficient. This dynamic load coefficient value has been verified on an improved impact machine implemented with a system to measure and record instantaneous dynamic stress disposed by the authors. The calculated values by the formula have been found to agree very well with the testing data recorded on the improved impact device. The formula is therefore useful in this type of dynamic stress analysis.