It depends on the gravitational field that the mass of marbles is under to experience a force of 0.5lbs.

lbs is a measure of force, then you're trying to compare that to measurements of mass without specifying the acceleration your marbles are under - which is needed to equate the units.

This MUST be an american question who never did get to grips between the difference of mass and weight. Standardize on your units, you can't mix mass and weight the way you're doing it.

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