# Wave Motion - Mission WM2 Detailed Help Which three of the following are units of frequency? List all that apply ... . Definition of Frequency: The frequency of a wave refers to the number of back-and-forth vibrational cycles that a point on the medium makes per unit of time. A wave involves the repeated and periodic vibration of a medium. As each particle of the medium undergoes its up and down (or back and forth) vibration, the familiar wavelike pattern is established in the medium. The frequency of a wave refers to how often the particles of the medium vibrate back and forth about their resting position. Mathematically, frequency is the number of cycles per unit of time. As such, the units of frequency will have the form of cycles/time or waves/time or vibrations/time. The standard metric unit of frequency is the Hertz (abbreviated Hz) and has the meaning of per second. In referring to a frequency as being 10 Hz, it is assumed that there are 10 repetitions (cycles, waves, vibrations, etc.) occurring per second. Many students confuse the speed of a wave with the frequency of a wave. Both are time-based quantities. As such, they often confuse their units as well. But don't be fooled! Speed refers to how fast the wave travels or the distance traveled per unit of time. As such, units on speed are distance units per time units. This is different than the units of repetitions per second which is typical of frequency units.  