A 0,51kg heavy stone is in free fall, how heavy is the stone?
Thanks in advance
You still haven't answered the basic question, "what do you mean by "heavy", here?"
One way of defining "heavy" is the object's weight: "the force of gravity on an object" which is always "mg", the mass times the acceleration due to gravity. Since you give the mass in kg, use g= 9.81 m/s^2.
But more common is the force with which an object is pressing down on a floor, scale, your hand, etc. Here, since the object is not pressing down on anything there is no force- its "heaviness" is 0.