To summarize: 1. The solenoid is supplied with an AC voltage. 2. Produces an AC magnetic field out of the page 3. Exerts an AC magnetic force on the Nb-Plate that's placed just outside of one end of the solenoid 4. There is a fluid circulating in the torus, with angular momentum L in the z-direction (out of the page) 5. What is the torque exerted on L due to the oscillating Nb-plate which forces the torus to oscillate with it? Directions: ∅ (Left-right), θ (In-Out of page) I'm not sure how they got an extra θ0 term in their torque about ∅. I know in small oscillations, the magnitude of change in L is ΔL ≈ θ0L Alternatively, considering change within half a period = T/2: ΔL/Δt = 2θ0L/T = 2fθ0L which also doesn't match their expression..