Objects float or sink depending on how heavy they are for their size compared with the liquid they’re in, and on how much upward push (buoyant force) the liquid can provide.

Quick Scoop

When you drop something into water, two main things decide its fate:

  • How dense it is (how much mass is packed into a certain volume).
  • How strong the water’s upward push is compared to the object’s weight.

If the water can push up as much or more than the object’s weight, it floats; if not, it sinks.

The Core Idea: Density

Density is “mass per unit volume” – basically how tightly matter is packed.

  • Objects less dense than water float (like many woods, plastic balls, a football).
  • Objects denser than water sink (like most stones, coins, and many metal tools).

Example:

  • A wooden block and a stone of similar size: the wood has lower density than water, so it floats; the stone has higher density, so it sinks.

The Hidden Player: Buoyant Force

When you put an object in water, it pushes water out of the way (displaces it).

  • The displaced water creates an upward force called buoyant force.
  • If the weight of the displaced water = the weight of the object, the object can float.
  • If the object weighs more than the water it displaces, it sinks.

So:

  • Float: buoyant force ≥ object’s weight.
  • Sink: buoyant force < object’s weight.

Why Heavy Things (Like Ships) Can Float

This is where shape and “average density” come in.

  • A solid steel block sinks because it’s very dense and doesn’t displace enough water before going under.
  • A steel ship, however, is mostly hollow and full of air, so its overall (average) density is less than water.
  • Because of its wide, hollow shape, it pushes aside a huge volume of water, creating enough buoyant force to balance its weight, so it floats.

Same reason:

  • A football floats: plastic shell + lots of air = low average density.
  • If you fill that football completely with water, it becomes much denser and likely sinks.

Why Some Things Float at First, Then Sink

Porous or airy objects can change from floating to sinking when water gets inside.

  • Dry sponge or porous wood: full of air pockets → lower density → can float.
  • Once soaked: water fills those air spaces → density increases → it may sink.

This is similar to:

  • A boat with holes: it starts to take on water, loses its trapped air, becomes denser overall, and can eventually sink.

A Simple Way to Picture It

Imagine water as a crowded swimming pool full of people:

  • Dropping an object in is like trying to step into the crowd. The object has to push some “people” (water) aside to make room.
  • The more water it pushes aside, the bigger the “push back” from the water.
  • If that push back (buoyant force) is strong enough to hold the object’s weight, it stays up; if not, it goes down.

Bottom line: Some objects float while others sink because of differences in density and the balance between their weight and the upward buoyant force from the liquid.

TL;DR:
If an object’s overall density is less than the liquid, and it can displace enough liquid to match its own weight, it floats; if it’s denser and can’t displace enough, it sinks.

Information gathered from public forums or data available on the internet and portrayed here.