Is the gravitational force greater on a 1-kg piece of iron or on a 1-kg piece of glass? defend your answer?

It is the same, there is no difference.

In fact, the force of gravity on an object is equal to the weight of the object, defined as

where m is the mass of the object and g=9.81 m/s^2 is the acceleration of gravity, As we can see, the magnitude of this force depends only on the mass of the object: in this problem, both the objects (the piece of iron and the piece of glass) have same mass (1 kg), therefore they have same weight, and same force of gravity.

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