I have an app with large boxes falling on top of the small red box. I would like to know when the small red block reaches a certain weight (X blocks are resting on top of it). I couldn't find a weight property for the red block. Any suggestions? 
EDIT: Just to clarify. The boxes falling from the top will be random sizes, and falling from random positions. So there isn't really a way to keeping track of what landed on top of the red block. I need some way to measure the downward force being applied to the red block