assumption phy

Physics principles and theories are just mathematical representation of reality. They could not perfectly describe the physical world. Hence, there must be assumption(s) associated with each and every principle/theory.

You need to give any assumptions you made in your calculation when using a physics principle, unless the assumption is "generally assumed"(i.e. apparent) or given in the question.

To your specific questions:

1.air resisatance 係唔係等於gravity
No. Air resistance is completely different from gravity. They are forces of different origins.

2. 如果係，題目寫assume no air resistance，gravity 係唔係10.
The acceleration due to gravity at sea level is g (with value about 9.81 m/s2, or roughly 10 m/s2), which is independent of air resistance.

It is only that when an object falls freely in air, it is acted upon by both air resistance and gravity. If air resistance is negligible, the object will fall under gravity with an acceleration equals to g.