Magnesium oxide is a binary ionic compound from its formula mgo how do you know that mg is the metal

Answer is: because it is first.
Oxide is a chemical compound that contains at least one oxygen atom and one other element. In oxide oxidation number of oxygen is -2. So in magnesium oxide oxidation number of magnesium is +2 (metal form cation).
When writing molecular formulas, metal is written first (because positive oxidation number).

The objects are acted upon by only one force. gravity. mass times the acceleration of gravity (-9.81meters/second squared).