BuoyancyYear 7 Science - Forces

Learning ObjectivesBy the end of this topic, you will:

- be able to define the terms buoyancy, density, and displacement.- How boats float- know that objects float in fluids they are less dense than- be able to state the factors that affect buoyancy- be able to determine an appropriate relationship between density and buoyancy

BackgroundWhether an object floats or not depends on several things.

Solid objects, like wood or apples, float because they have lots of air in them and are lighter than the water aroundthem.

Objects like coins or paper clips sink because the atoms in them are close together and they are heavier than water.

BuoyancyObjects like boats float, even if the metal that makes up their hull is heavier than water, because of a force called buoyancy.

That means the water pushes up against them.

You can feel this force by holding a block of wood under water.

How does it feel to your hand? It almost feels like thewood is struggling against you to get to the top of the water.

That push is buoyancy.

Example with BreadIf the metal in a ship was all squashed into a ball, it would sink.

Because a boat is built to spread out the metal over a wide area, it floats because the water pushes up on a lot of the metal.

Remember your last swimming adventure. If you wrapped your arms around your legs and curled up into a ball, did you sink?

When you stretched out flat, did you float? That’s because more water pushes against you since your body is laid out flatter.

When you curl up into a ball, less water is pushing against you.

DisplacementAnother thing that determines if an object will float is its displacement, or the amount of water it pushes away.

When you get in the bathtub, the water rises because your body pushed some of the water away.

An object will float if it is lighter than the water it pushes away.

The heavy metal in a ship is formed into a hull, which is basically a metal shell filled with air.

The metal and air together are lighter than the water they push away, and a heavy ship will float.

Your Turn!Watch the video below on your iPad and answer the questions that are embedded in the video.

https://youtu.be/VDSYXmvjg6M

Learning ObjectivesTake a moment to summarise what you have learned.

You should:

- be able to define the terms buoyancy, density, and displacement.- know how boats float- know that objects float in fluids they are less dense than- be able to state the factors that affect buoyancy- be able to determine an appropriate relationship between density and buoyancy