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@Z0IDBERG, it’s from calculus. When you perform an integration of an equation without a defined domain so it gives you a new equation. The +C is representative of a constant applied because the equation can be anywhere and a specific constant will be required to correct based on where the domain is set
Well the +C is there because when you derive an equation any constant is lost so when you go back and integrate there is possibly an unknown constant that you must solve for. And sometimes air resistance can be stated as negligible but for any real calculation you shouldn't ignore it.