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

Forces in water upthrust friction thrust weight The force that pushes an object forward. What is thrust? The force that pushes an object upwards in a liquid or a gas, also called buoyancy. What is upthrust? upthrust friction thrust weight

Ferry forces

Changing forces The ferry arrives at Calais and is loaded with travellers. 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 increases both equal

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?