Does the phent_setmass()-function use this formular to calculate the mass of a box?:

code:

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mass_amount = mass*height*width*length; //mass is a parameter of phent_setmass()

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The parameter you specify with phent_setmass is total mass, not density. I.e. A6 solves

code:

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mass = density*height*width*length;

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with mass being your parameter and density the unknown.

when in doubt about a function, I always check the units.

For instance, your formula:

mass_amount = mass*height*width*length;

would produce:

mass_amount = kilograms*meters*meters*meters

or

mass_amount = kg*m^3 which is not a unit of mass

just something to always check when you are not sure. Unit analysis is a very powerful thing.