Lesson 2 · 12 min
Waves on a line
A disturbance that travels. Frequency is a source property; speed is a medium property.
The three-way identity
v = fλ. Wave speed is set by the medium (tension and linear density on a string; bulk modulus and density in a fluid). Frequency is set by whatever is shaking it. Wavelength then has to be v/f. You do not get to pick all three.
v = fλ v_string = √(T/μ)
μ = mass per unit length. Tighten a guitar string (T up) and v up — for a fixed length, f up.
Energy, not bulk matter
The wave carries energy and information. The bits of string mostly go up and down. This is why you can send a signal down a cable, or a pressure pulse down a pipe, without sending the copper or the water along with it.
Check
A 2.0 m string, μ = 5.0 g/m, under 80 N. Wave speed?