The figure shows a snap shot of a vibrating string at t = 0. The particle P is observed moving up with velocity 20√3cm/s. The tangent at P makes an angle 60∘ with x-axis.
The speed and the direction of the wave is
20 cm/s along negative x-axis
dydx=tan60∘=√3, vp=−vdydx⇒20√3=−v√3
⇒ v=20 cm/s