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Newton's second law of motion states that force equals mass times acceleration: F = ma. If you know the net force on the object, and it's acceleration, you can rearrange this formula to find the mass: m = F / a. Force is measured in N (newton), which you can also write as (kg * m)/ s2. Acceleration is measured in m/s2. When you calculate F / a, the units cancel to give you an answer in kilograms (kg). Mass is the amount of matter in an object; this does not change unless you cut off part of the object, or attach more material. Weight is a measurement of gravity's effect on mass. If you move the object to an area with different gravity (such as from the earth to the moon), it's weight will change, but it's mass will not. An object with more mass does weigh more than an object with less mass, if they're experiencing the same gravity. . If you're doing your chemistry homework, you may come across the term "molar mass." This is a related concept, but instead of measuring an object, you measure exactly one mole of a substance. Here's how to calculate it in most contexts:  For an element: look up the atomic mass of the element or compound you are measuring. This will be in "atomic mass units" (amu). Multiply by the molar mass constant, 1 g/mol, to put it into standard molar mass units: g/mol. For a compound: add the atomic masses of each atom in the compound to find the total amu of the molecule. Multiply this total by 1 g/mol.
Determine mass with force and acceleration. Understand mass and weight. Calculate molar mass