Final answer:
Garrett is measuring the frequency of the waves, which is the number of waves reaching a point per unit time. Frequency is closely related to the wave's period, and it is measured in hertz (Hz).
Step-by-step explanation:
When Garrett counts the number of waves that reach the shore over a 10-second period, he is measuring the waves' frequency. Frequency is the number of complete waves that pass a point in a certain period of time and is expressed in units called hertz (Hz). The frequency is calculated using the formula f = 1/T, where f is the frequency and T is the period of the wave, which is the time for one complete oscillation or cycle of the wave motion.
Simplified mechanical waves, which repeat themselves in simple harmonic motion, can be represented by sine or cosine functions. As these waves move across the water, they cause objects on the water's surface, like a seagull, to experience simple harmonic motion, bobbing up and down as the wave crests and troughs pass by.