we call this to be the x ′ –y ′ plane (coincident with the plane of the page). The particle at P describes a circular path of radius r with centre C on the axis; CP = r . In time ∆ t , the point moves to the position P ′ . The displacement of the particle d s , therefore, has magnitude d s = r d θ and direction tangential at P to the circular path as shown.
Here d θ is the angular displacement of the particle, d θ = P CP ∠ ′ .The work done by the force on the particle is d W = F . d s = F d s cos φ = F ( r d θ )sin α