Gravitational mass is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass this is typically done with some short of balance scale the beauty of this method is that no matter where,or what planet ,you are,the masses will always balance out because the gravitational acceleration on each object will be the same 
inertial mass is found by applying a known force to an unknown mass ,measuring the acceleration ,and applying    newtons second law .this gives as accurate a value for mass as the accuracy of your measurements .when astronauts need to be weighed in outer space ,they actuality find their internal mass in a special chair .
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