Standing waves are set up on the apparatus. Here the distance from P to Q is L=1.20m and the oscillator is set to a frequency 120Hz. A standing wave appears when the mass of the hanging block is 286.1g or 447.0 grams, but not for any intermediate mass. What is the linear density of the string? I know μ = tension/v^2 = tension/(λ^2 f^2) I need to get lambda but I don't know how to. help is greatly appreciated!!