The nature of normal grain growth in the presence of a second phase deserves special consideration. The moving boundaries will be attached to the particles exert a pulling force on the boundary restricting its motion. Therefore if the boundary intersects the particle surface at 90° the particle will feel a pull of (2πrγcosΘ)*sinΘ. This will be counterbalanced by an equal and opposite force acting on the boundary. As the boundary moves over the particle surface Θ changes and the drag reaches a maximum value, this happens when Θ becomes______

As the boundary moves over the particle surface Θ changes and the drag reaches a maximum value when sinΘ*cosΘ is a maximum, i.e. at Θ = 45°. The maximum force exerted by a single particle is therefore given by πrγ.