2. No matter what the direction of the displacement the force always acts in a direction to restore the system to its equilibrium position. Such a force is called a" restoring force(恢复力) 3. The number of cycles per unit time is called the ●“ frequency (17-1) Unit: period(s) frequency (Hz, sI unit), 1 Hz =1 cycle/s 4. The magnitude of the maximum displacement from equilibrium is called the amplitude of the motion2. No matter what the direction of the displacement, the force always acts in a direction to restore the system to its equilibrium position. Such a force is called a “restoring force(恢复力)”. 3. The number of cycles per unit time is called the “frequency” f. (17-1) Unit: period (s) frequency(Hz, SI unit), 1 Hz = 1 cycle/s T f 1 = 4. The magnitude of the maximum displacement from equilibrium is called the amplitude of the motion