5.6 Floating & Sinking

Drop a coin in water and it sinks; drop a wooden block and it floats. Gravity pulls down on both — so why doesn’t everything sink? There must be another force, pushing up.

Activity 5.13 — Feel the upward push Push a tightly closed empty bottle down into a bucket of water. You feel water shoving it upward, and the instant you let go it bounces back to the surface. The water is clearly applying an upward force on the bottle.

Buoyant Force (Upthrust) The upward force a liquid applies on an object placed in it is called the buoyant force or upthrust. Every object in a liquid feels two forces: gravity pulling it down (its weight) and the buoyant force pushing it up.

  • If the gravitational force is greater than the buoyant force → the object sinks.
  • If the two forces are equal → the object floats.

The buoyant force depends on factors including the density of the liquid (which you’ll study in a later chapter).

Try the tug-of-war below: set how strong gravity (the object’s weight) and the buoyant force are, and see whether the object floats or sinks.

Float or sink?
A contest between weight (down) and buoyant force (up).



A Step Further: Archimedes’ Principle The Greek scientist Archimedes discovered that an object in a liquid is pushed up by a force equal to the weight of the liquid it displaces. So if an object displaces liquid weighing less than itself, it sinks; if it displaces liquid weighing as much as itself, it floats. A huge steel ship floats because its broad hull pushes aside an enormous weight of water.

Ever Heard Of… A Rock That Floats Pumice is a rock that floats! It forms when gas-rich lava cools fast during a volcanic eruption, trapping countless tiny bubbles. The result is a light, porous rock full of air pockets — less dense than water, so it floats.