---
title: "Floating and Sinking"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 5
exercises: 9
source: https://one-course.com/books/physics/1/en/chapter/5-floating-and-sinking
---

# Chapter 5 — Floating and Sinking

Drop a pebble in a puddle: it disappears to the bottom. Drop a leaf: it rides on top like a little boat. Bath time, puddles and ponds all ask the same question — what [floats](#def-g1-floating-and-sinking-words), what [sinks](#def-g1-floating-and-sinking-words), and can we guess before we drop it in?

## 5.1 Float or sink?

**Definition 5.1 (Floating and sinking).**

A thing *floats* when the water holds it up at the top; it *sinks* when it goes down to the bottom. A cork floats; a pebble sinks.

**Method 5.2 (A fair float test).**

1. Fill a big bowl (or the sink) with water;
2. put the object on the water *gently* — no throwing: a splash can push even a floater under for a moment;
3. let go completely, and wait while you count to $5$ ;
4. look: on top is a floater, at the bottom is a sinker.

Test each object the same way, so the comparison is fair.

**Example 5.3 (Try it: the sorting game).**

Gather a cork, a coin, a plastic spoon, a metal spoon, a pebble, a pencil and a dry leaf, and test them one by one with [Method 5.2](#met-g1-floating-and-sinking-test). Floaters: the cork, the plastic spoon, the pencil, the leaf. Sinkers: the coin, the metal spoon, the pebble. Before each drop, say your guess out loud — that is what scientists call a *prediction*, and being wrong is half the fun.

![After the sorting game: floaters ride the top of the water, sinkers rest on the bottom.](https://one-course.com/images/onecourse/chapters/physics-1/g1-floating-and-sinking/fig-34426bd4ccdf.svg)

*After the sorting game: floaters ride the top of the water, sinkers rest on the bottom.*

![An apple floating in a bowl of water: it rides low, most of its body under the surface — yet it floats.](https://one-course.com/images/onecourse/chapters/physics-1/g1-floating-and-sinking/img-db2e24d4cf6b.jpg)

*An apple floating in a bowl of water: it rides low, most of its body under the surface — yet it [floats](#def-g1-floating-and-sinking-words).*

## 5.2 Heavy is not the whole story

Many people guess “heavy things [sink](#def-g1-floating-and-sinking-words), [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) things [float](#def-g1-floating-and-sinking-words)”. The water has a surprise for them.

**Example 5.4 (The log and the coin).**

A fallen tree trunk is far too heavy for you to lift — yet it [floats](#def-g1-floating-and-sinking-words) down the river. A little coin weighs less than a cookie — yet it [sinks](#def-g1-floating-and-sinking-words) straight away. So heavy-or-light cannot be the whole rule: the huge log [floats](#def-g1-floating-and-sinking-words) while the tiny coin drowns.

**Example 5.5 (It is the stuff that counts).**

Look back at the sorting game: the plastic spoon floated and the metal spoon sank, though they have the same shape and the same job. What made the difference is the *stuff* they are made of — wood and cork and most plastics are floating stuffs; metal, stone and glass are sinking stuffs. A thing usually behaves like its stuff: wood [floats](#def-g1-floating-and-sinking-words) whether it is a twig or a whole trunk.

**Remark 5.6 (A pinch of mystery).**

*Why* does wood [float](#def-g1-floating-and-sinking-words) and metal [sink](#def-g1-floating-and-sinking-words)? That is a real question, and its full answer is one of the treasures of this course — it needs ideas that arrive in later years. For now we collect good observations; the “why” will find us ready.

## 5.3 The water pushes back

**Example 5.7 (Try it: push the ball under).**

Take a floating ball in the bath and push it slowly underwater with your palm. Feel that? The water pushes it back up, harder the deeper you push — and if your hand slips, the ball leaps out of the water. The water is not just *letting* floaters sit on top: it is actively holding them up, like your hands holding a baby.

![A ball held underwater: your hand pushes down, the water pushes up. Let go, and the water’s push wins.](https://one-course.com/images/onecourse/chapters/physics-1/g1-floating-and-sinking/fig-45a5bc33411b.svg)

*A ball held underwater: your hand pushes down, the water pushes up. Let go, and the water’s push wins.*

**Example 5.8 (The plasticine boat).**

Roll a lump of plasticine into a ball and drop it in the water: it [sinks](#def-g1-floating-and-sinking-words) — plasticine is a sinking stuff. Now take the *same* lump and shape it into a little bowl with high sides, and set it gently on the water: it [floats](#def-g1-floating-and-sinking-words)! Same stuff, same lump — only the shape changed. A wide, hollow shape gives the water more to push up on. That is the secret of the great ships: metal is a sinking stuff, but a metal hull is shaped like a huge hollow bowl.

![One lump of plasticine, two shapes, two fates. The hollow bowl shape gives the water more to hold up.](https://one-course.com/images/onecourse/chapters/physics-1/g1-floating-and-sinking/fig-06a611ac89d0.svg)

*One lump of plasticine, two shapes, two fates. The hollow bowl shape gives the water more to hold up.*

## 5.4 Exercises

**Exercise 5.1 ★.**

Sort into floaters and sinkers: a cork; a key; a dry twig; a marble; a plastic duck; a stone.

**Solution of Exercise 5.1.**

Floaters: the cork, the dry twig, the plastic duck. Sinkers: the key, the marble, the stone.

**Exercise 5.2 ★.**

In [Method 5.2](#met-g1-floating-and-sinking-test), why must you put the object on the water gently instead of throwing it in?

**Solution of Exercise 5.2.**

A splash can push even a floater underwater for a moment, so a thrown object might fool you. Putting it down gently and waiting makes the test fair.

**Exercise 5.3 ★.**

Before testing, Nora guesses: “The metal spoon will [float](#def-g1-floating-and-sinking-words) because spoons [float](#def-g1-floating-and-sinking-words).” What happened in the sorting game? What should Nora look at instead of the shape’s job: the stuff or the name?

**Solution of Exercise 5.3.**

The metal spoon sank while the plastic spoon floated. Nora should look at the *stuff* a thing is made of, not its name or its job.

**Exercise 5.4 ★.**

Give one thing that is heavy and [floats](#def-g1-floating-and-sinking-words), and one thing that is [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and [sinks](#def-g1-floating-and-sinking-words). What does this show about the guess “heavy things [sink](#def-g1-floating-and-sinking-words)”?

**Solution of Exercise 5.4.**

Heavy and floating: a tree trunk (or a ship). [Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and sinking: a coin (or a key). So “heavy things [sink](#def-g1-floating-and-sinking-words)” is a bad rule — weight alone does not decide.

**Exercise 5.5 ★.**

A twig [floats](#def-g1-floating-and-sinking-words). Will a whole tree trunk [float](#def-g1-floating-and-sinking-words) too? Which example of the chapter helps you answer?

**Solution of Exercise 5.5.**

Yes — wood is a floating stuff, and a thing usually behaves like its stuff, twig or trunk. The log of [Example 5.4](#ex-g1-floating-and-sinking-log) [floats](#def-g1-floating-and-sinking-words) down the river.

**Exercise 5.6 ★.**

Push a floating ball underwater in the bath and let go. What do you feel while pushing, and what does the ball do when you let go?

**Solution of Exercise 5.6.**

While pushing, you feel the water pushing the ball back up, harder the deeper it goes. When you let go, the ball pops back up to the top — the water’s push wins.

**Exercise 5.7 ★.**

Ships are made of metal, and metal is a sinking stuff. What shape saves the ship? Which experiment of the chapter shows it?

**Solution of Exercise 5.7.**

The hollow-bowl shape: a wide hull with high sides gives the water more to push up on. The plasticine boat of [Example 5.8](#ex-g1-floating-and-sinking-boat) shows it — same stuff, but the bowl shape [floats](#def-g1-floating-and-sinking-words).

**Exercise 5.8 ★.**

Test at bath time: does a full, closed bottle of water [float](#def-g1-floating-and-sinking-words) or [sink](#def-g1-floating-and-sinking-words)? And the same bottle empty, with its cap on? Say your guesses first, then try.

**Solution of Exercise 5.8.**

The full closed bottle [floats](#def-g1-floating-and-sinking-words) low in the water (just barely, for most bottles); the empty capped bottle [floats](#def-g1-floating-and-sinking-words) high, riding on the water — it is mostly air inside.

**Exercise 5.9 ★★.**

Leo’s plasticine boat [floats](#def-g1-floating-and-sinking-words) nicely — until he squashes it back into a ball, and it [sinks](#def-g1-floating-and-sinking-words). “The water got tired of holding it,” says Leo. Give a better explanation, using [Example 5.8](#ex-g1-floating-and-sinking-boat).

**Solution of Exercise 5.9.**

The water never gets tired. Squashed into a ball, the plasticine gives the water only a small round bottom to push on; shaped as a hollow boat, the same lump spreads wide and the water’s push on all that bottom is enough to hold it up. It is the shape that changed, not the water.