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.
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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.
Upthrust, defined
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
The upthrust equals the weight of the fluid displaced. An object displaces 3 kilograms of water, and the gravitational field strength is 10 newtons per kilogram. What is the upthrust, in newtons?
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
The upthrust equals the weight of the water displaced. For water, plot the upthrust as the displaced mass rises: at 1, 2, 3 and 4 kilograms, with a gravitational field strength of 10 newtons per kilogram.
Float or sink?
Put the steps to decide whether an object floats in a sensible order.
- Find the weight of the object
- Work out the weight of the fluid it can displace
- That displaced weight gives the upthrust
- Compare the upthrust with the weight
- It floats if the upthrust can equal the weight
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.
True or not?
Tap the TWO statements that are TRUE about floating and sinking.
- Upthrust equals the weight of the fluid displaced.
- An object floats when the upthrust equals its weight.
- Only very light objects can ever float.
- Upthrust always acts downwards.
- Floating depends on density alone, never on the forces.
Will it float?
Use upthrust and weight to decide each case.
- An object weighs 20 newtons. Fully in water it would feel an upthrust of 25 newtons. What happens?
- An object weighs 30 newtons but can only ever get 18 newtons of upthrust. What happens?
- Why can a heavy steel ship float when a small steel bolt sinks?
Explain it
Explain, using upthrust and weight, why some objects float and others sink.
- Say what upthrust is
- Explain what the upthrust is equal to
- Explain when an object floats and when it sinks
- Give one example of a floating object and why it floats