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Learning Objective:To investigate how ships float.

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What will happen to the

paper clip when it is dropped in the

water?

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Because the metal is more

dense than water, it will sink to the

bottom.

Paperclips are made of steel. This metal has a much higher density than water.

Then how did the Titanic, a ship made from steel which weighed over 50,000

tons, float on water?

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Think, pair, share your ideas.

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It’s all to do with water displacement and the ship’s shape.

Let me introduce you to a great Greek mathematician, inventor and

discoverer of water displacement: Archimedes.

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The story goes that Archimedes ran a bath until it was was full to the brim.

As he got in, he noticed the water overflowed. The effect of the water level rising as he got in the bath is called water displacement. This

sparked his curiosity and, after a few more tests and measurements, he

found that the amount of water that overflowed had the same volume as

the object that was placed in the water.

In the story, it is said that Archimedes was so excited by his discovery that he ran down the street shouting ‘Eureka!’

which is Greek for ‘I found it!’. www.planbee.com

Archimedes had been tasked by important people to determine whether a crown had been made of

pure gold or if it had had been made out of cheaper metals. He couldn’t melt the crown down, so instead he used his ideas of water

displacement. By measuring how much water the crown

displaced, and how much it weighed, he could work out how dense the material was. If it had a lower density than a similar amount of gold, he

could determine the crown was made of cheaper and less dense materials.

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Archimedes continued to study water displacement which he linked to the

science behind why objects float.He studied the force in the water which

pushes against the force of gravity called buoyant force (or upthrust).

This buoyant force is equal to the amount of water that the object displaces.

Buoyant force

Weight of water

displaced=

In other words, if the force of gravity (which affects the object’s

weight) is not as strong as the buoyant force, the object will float…

Let’s break it down a bit more…

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The Arcimedes Principle states:

If the weight of the object being placed in the water is less than the weight of the water

being displaced, it will float.

50g

The weight of this displaced water is equal to the

buoyant force which is pushing the block up,

against the force of gravity. Because the weight of the displaced water is more

than the weight of the block, the block floats.

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This block was weighed and put into a container which was full to the brim with water. The displaced water (which overflowed the container) was

collected and weighed.

35g

70g

Can you explain what is happening in this example?

Try to use the words buoyant force and displaced water in your explanation.

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A different block was weighed and put into the container which was full to the brim with water. The displaced water (which overflowed the container)

was collected and weighed.

150g

70g

More water is displaced this time. This means that

the buoyant force is stronger than the last

example. However the block weighs more than the

displaced water. This means the gravitational pull is stronger than the

buoyant force and the block sinks.

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150g

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So, now we know that if an object is heavier than the water it displaces, it will

sink.

We also know that the more water that is

displaced, the more powerful

the buoyant force will be, pushing the object up.

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A ship like the Titanic needs to displace a

LOT of water to have enough buoyant force

in order to float. This is achieved

through the clever design of the hull.

Every ship will have a carefully measured maximum amount of weight allowed on board to make sure that the balance between

the weight and buoyant force doesn’t tip the wrong way!

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A ship’s hull (the bottom part of the ship) is designed to displace as much water as

possible. They are usually made wide and hollow with high sides, which increases the amount of water they displace and therefore increases the buoyant force.

A ship’s hull is made hollow, rather than

being a solid block of metal. This reduces

the weight of the ship, which reduces

the amount of buoyant force

needed to keep it afloat.

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By user:Wranzl - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1141176

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Can you complete this sentence?

A ship can float because….

• Its overall weight is less than the weight of the water it displaces, meaning the buoyant force is stronger than the gravitational pull acting on the ship.

• Its hull is designed to displace as much water as possible to increase the buoyant force acting upon the ship.

• Its hull is designed to be hollow, making it lighter than the water it displaces.

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Was your answer similar to any of these?

Try to use ‘water displacement’, ‘buoyant force’ and ‘weight’ in

your explanation.

The Titanic sank because the iceberg she hit created

holes in the side of the ship and water began to fill the

hull.Think, pair, share...

Can you use what you have learnt today to explain why

taking on water would cause the ship to sink?

Plenary

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The hole in the side of the hull meant that the ship was no longer displacing water effectively. This meant that the buoyant force was

slowly weakening.At the same time, taking on water

meant that the ship’s overall weight was slowly increasing as

the air was pushed out by the water. Eventually the buoyant force

was not enough to keep the ship afloat.

Plenary

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