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Float or Sink?

A tiny coin sinks, yet a huge steel ship floats. Meet upthrust, the upward push of a fluid, and learn why an object floats when the upthrust balances its weight.

⏱️ 16 min 🎯 14 activities Teachers Not yet rated Students Not yet rated

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What you'll cover

Why do ships float? 🚢

A tiny metal coin sinks, yet a huge steel ship floats. The secret is **upthrust**, the upward push a fluid gives to anything placed in it. This module is about upthrust, and why some things float while others sink.

Words to know 🔑

Four ideas run through floating and sinking:

What is upthrust? 🎯

What is upthrust?

  • The upward force a fluid gives to an object
  • The downward pull of gravity on an object
  • A sideways push from the wind
  • The friction between two solid surfaces

Float or sink? ⚖️

Whether something floats comes down to two forces: its weight, and the upthrust.

Up or down? 🔗

  • A huge steel ship
  • A small metal coin
  • A helium party balloon
  • A heavy stone
  • Floats: its shape displaces a lot of water
  • Sinks: its weight beats the small upthrust
  • Rises: the air upthrust beats its weight
  • Sinks: it cannot displace enough water

The key rule 💧

Here is the key rule: the upthrust on an object equals the WEIGHT OF THE FLUID it displaces. An object floats when it can displace a weight of fluid equal to its own weight. A steel ship floats because its wide shape pushes aside a huge weight of water.

Find the upthrust 🔢

An interactive activity.

True about floating? ✅

Which THREE statements about floating are TRUE?

  • An object floats when the upthrust equals its weight
  • Upthrust equals the weight of the fluid displaced
  • A ship floats by displacing a large weight of water
  • Only very light objects can ever float
  • Floating depends on size alone

Upthrust and displaced water 📈

An interactive activity.

Float or sink? 🪜

An interactive activity.

Complete the idea 🧩

Upthrust is the _____ force a fluid exerts on an object. It equals the weight of the fluid _____. An object _____ when the upthrust equals its weight, and sinks when its _____ is greater. To decide, compare the forces, not the _____ on its own.

upward displaced floats weight density downward added sinks mass colour

True or not? 🖍️

An interactive activity.

Will it float? 🧭

An interactive activity.

Explain it ✍️

An interactive activity.