Why is one kilogram of iron heavier than one kilogram of cotton?
One kilogram of iron and one kilogram of cotton have the same mass and weigh the same. Mass is a measure of the amount of matter in an object, and it is the same regardless of an object's location or the forces acting on it. The weight of an object, on the other hand, is the force exerted on the object due to gravity and depends on the mass of the object and the acceleration due to gravity at a particular location.

So, a kilogram of iron and a kilogram of cotton have the same mass and will weigh the same on a scale, but they will feel different in our hands because of their densities. The density of an object is its mass per unit volume, and it is a measure of how closely the mass of an object is packed together. Iron has a much higher density than cotton, which means that the same mass of iron takes up less volume than the same mass of cotton. This makes iron feel heavier than cotton, even though they have the same mass.