That's how to calculate the force of a falling object, calculating the force of a snakes' strike isn't the same.
You could use Newtons Second Law of Motion: F = ma (Force = mass x acceleration) Where m = the mass of the parts of the snake in motion and the acceleration is the second derivative of the position function of the snake while in motion.
Due to the potential for error in these measurements this will likely only yield an approximation, but it should at least give you an idea.
The simplest way to find out would be to have a snake strike a force plate and get a reading, but most people don't just have one of those laying around.