A string, tied to a sinusoidal oscillator at P and running over a support at Q, is stretched by a block of mass m. Separation L = 1.20 m , linear density μ = 1.6 g/m , and the oscillator frequency f = 120 Hz . The amplitude of the motion at P is small enough for that point to be considered a node. A node also exists at Q. What mass m allows the oscillator to set up the fourth harmonic on the string?