I am trying to teach myself Physics, so I can use it in engineering for my own devices. The project I am working on right now is a very simple spring based launcher. This is frustrating me to no end. Here is my objective. I want to lunch a golf ball about 10 or so feet. I know that I need to calculate the springs energy. I also know that the springs energy is approximately equal to kinetic energy. All I am doing is transforming spring energy into kinetic energy. The equation is like so.. 1/2ks2 approx = 1/2(Mball + Mpiston)V2 The ball is about 0.101lb. In order for me to proceed, I need to know the mass of the piston. I have the other information I can work that out, however... How the hell would I do that if I have not constructed the piston yet? I looked up how to find the mass. The information I found goes on and on about density and so on. The piston is going to be made out of Metal, more than likely steel. The head is going to be steel and the rod is going to be steel. Now I am following a tutorial on how to go about constructing this device. Problem is, his measurements are different than mine. Because I have something else in mind for my luncher. For example, my spring is going to be 9.5in long with a maximum weight capacity of 27lb. Obviously, I assume, I am going to overshoot the 10-foot mark. His is half that or so. I need to know the mass of the piston. How would I find that, if I have not constructed it? Do I just "estmate"