Question

A physical balance measures small differences in mass up to ______

Answer 1

Mass is the amount of matter in the object
(Mass and weights are two different things
 Mass is the amount of matter in the object
 Weight is the effect of gravity's effect on something
 Put it simply if you change planets your mass will remain the same but    your    weight will be different)
Now as the questions asks,
A physical balance can measure mass difference upto miligram

Answer 2

ANSWER:

A physical balance measures small differences in mass up to 0.001 milligram.

EXPLANATION:  

The smallest measurement of mass in a physical balance is weighs the mass and gravitational mass of the substance. It could measure the weight of a substance with a minimum difference of 0.001 mg.

As this is the least weight or mass of the substance for measurement, every objects in the surroundings could be measured in its mass up to microgram of weight.