Hmm, compression force, AFAIK there are no functions in the API for this purpose.
If the stacked boxes compress slightly, you could try measuring the distance between them or return the collision depth. Either could be useful to determine the amount of compression involved, giving some indication of the force. You may need to combine the mass/density with this info if the objects are of different sizes/weight.
If the stacked boxes compress slightly, you could try measuring the distance between them or return the collision depth. Either could be useful to determine the amount of compression involved, giving some indication of the force. You may need to combine the mass/density with this info if the objects are of different sizes/weight.