Mass is how much matter is in an object. It is used to measure two things: How much force gravity is exerting on an object and how much the object resists forces on it. Mass is often confused with weight. Weight is how much force gravity is exerting on you. Your weight is determined by the following formula: W=mg. w is weight, m is your mass, and g is your gravitational acceleration (what your acceleration would be if gracity were the only force acting on you. Close to Earth, g is 9.8 km per second. g is found by the formula g=M/r2 where M is the mass of the other object and r is the distance between you and the object. Mass is force/acceleration. Mass is used in a lot of formulas. Force is mass times acceleration; acceleration is force divided by mass.