If this is 3D model, the reference area is the important one and I can see you set this right.nBut now you are actually getting very high force. ... how large is the gab between the two spheres? nAlso if your cell height is Y+ , try to use smaller time step ... 0.001 is large ... try making this 0.0001 .. Also the first few timesteps would not be accurate ... if you plot the force you will see it oscillates around a value at the beginning until it diverges to a certain value. n