To measure the force required to break a shell I have a simple device in which a bolt rests on the shell attached through a piece of plywood. This piece of plywood slides through 4 bolts on the plywood on the bottom. There is a bucket on top which I fill with water at a constant rate until failure at which point I stop the water flow. Once calculating the mass being placed on the shell (plywood, bucket, water etc.) I am left with the mass required to break the shell. I was just wondering how to convert this into a measure of force needed to overcome the shell. I assume you use the F=MA equation but I'm unsure what you use for A. Thanks.