Floating and Density

Aims:

To know that when objects are immersed in water there is an upthrust on them.

To explain upthrust through experimental investigation.

To know what is meant by the term density

Density and floating

Starter: How could you use a bath, a measuring jug, a marker pen and some water to work out the volume of your body?

What happens to our weight when we float?

When you are standing on the ground gravity is pulling you down.

An upward force from the ground stops you sinking into the Earth

This upward force is called upthrust

In water you feel as though you weigh less because the upthrust of the water pushes against your weight

upthrust

friction

thrust

weight

What is thrust?

The force that pushes an object forward.

What is upthrust?The force that pushes an object upwards in a liquid or a gas, also called buoyancy.

Forces in water

Ferry forces

What happens to the weight of the ship?

What happens to the upthrust on the ship?

Which is the largest force - weight or upthrust?

increases

both equal

increases

The ferry arrives at Calais and is loaded with travellers.

Changing forces

Copy and complete this table- you have 10 mins to complete

the task

Object Weight in air (N)

Weight in water (N)

Density and floating

An object will float when the upthrust cancels out its weight

You can work out if an object will float if you know its density (how much mass there is in 1 cm3 of

something)

Calculating density

density = mass ÷ volume

To work out density you must make sure that you have measured…. The mass in grams The volume in cm3

The density of water is 1 g/cm3

If something has a density less than 1 g/cm3 it will float in water

What if you make the liquid heavier?

If the liquid is heavier than water then you do not need to push as much out of the way to get the same upthrust

You can make a liquid heavier by dissolving something in it

Now it is your turn

Without weighing the blocks predict if they will float or sink

Write your predictions down in a table then test them

Once you have tested your predictions calculate the density of the different blocks

Remember density = mass ÷ volume

Pete the Pirate wants to hide his treasure by sinking it to the bottom of the sea.

His treasure has a mass of 47kg.

It has a volume of 620 cm3.

Remember that a kg is 1000g.

Work out the density of Pete’s treasure.

Will it float or sink?