Abstract: An impact testing machine in which a striker held by a fixed mounting is struck by a striker carried by a pivoted arm. A motor, connected to the arm by a clutch, accelerates the arm at such a rate that the striker achieves useful speed within one revolution from stationary, and the clutch operates to disengage motor and arm just before the striker hits the specimen so that the momentum of strike is independent of the motor. Various constructions and arrangements of motor and arm are disclosed whereby the necessary high angular acceleration may be imparted to the arm, including a lost-motion connection using a reeled wire bearing upon a pulley.