Question

How does density work? Why does density allow a `100kg` block of ice to float on water, but a `1g` steel ball to sink in water? Or a helium ballon to rise in air but a piece of paper to fall?

Answer 1

density is a measure of how much a fixed volume weighs (or more accurately the "mass" of a fixed volume)

Explanation:

If an object is immersed in a medium, it displaces a volume of the medium equal to its own volume.

For example a `100` kg block of ice might have a volume of `1.4` cubic meters and if submerged in water it would displace `1.4` cubic meters of water;
the displace `1.4` cubic meters of water might have a density of `1.0` kilograms per cubic meter, so the mass (think "weight") of water displace would be `1.4` kilograms.

If the weight being displace is greater than the weight of the object, the displace volume will try to "sink" below the object causing the object to '"float".

If the weight of the object is greater than the weight of the displaced medium (as would be the case with the steel ball in water) the object will try to "sink".