Thank you sir 

Defining velocity to a body is now clear. 

Yes , mass of particles are included with plate. 

I have to calculate the  mass loss of the material (say plate) after the impact of the striking of some object (say balls) . here target material is flexible and striking object is consider as rigid body. Using the erosion expression in user defined result I am able to calculate the element loss but not getting the way to calculate the loss of material due to the impact of the particle. 

Erosion= mass of plate before the impact- mass of plate after the impact

With Regards