Buoyancy is the upward force exerted by a fluid that opposes the weight of an immersed object. Objects float if they are less dense than the fluid they displace. A boat floats even if its metal hull is denser than water because the hull is spread out over a wide area, increasing the volume of water it displaces. Displacement refers to the amount of fluid an object pushes aside when submerged, and an object will float if it displaces more fluid than its own weight.

1. Buoyancy
Year 7 Science - Forces

2. Learning Objectives
By 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

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

4. Buoyancy
Objects 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 the
wood is struggling against you to get to the top of the water.
That push is buoyancy.

5. Example with Bread
If 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.

6. Displacement
Another 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.

7. Your Turn!
Watch the video below on your iPad and answer the
questions that are embedded in the video.
https://youtu.be/VDSYXmvjg6M

8. Learning Objectives
Take 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