Why Do Things Float? What Would You Say?  D. Crowley, 2007

Why Things Float L.O: To understand why objects float

Floating  Look at the model boat below - how is it floating?  See if you can identify the forces involved, and add the force arrows (what size should these arrows be)?  In your book draw a diagram of the boat, add the force arrows and add a short explanation of why it floats.  Look at the model boat below - how is it floating?  See if you can identify the forces involved, and add the force arrows (what size should these arrows be)?  In your book draw a diagram of the boat, add the force arrows and add a short explanation of why it floats.

Floaters  So why is it some objects float, and others sink?  Look at the picture below of some rocks sinking. Explain why the rocks sink  So why is it some objects float, and others sink?  Look at the picture below of some rocks sinking. Explain why the rocks sink Gravity Reaction Force - up thrust of water This time the reaction force is much smaller than that of gravity - this means that the rock will accelerate into the depths…

Experiment  So far we have found that some objects float, and others sink  There must be a pattern to which ones float, and which sink - can you work it out?  Is it that all heavy objects sink, and all light ones float? Does it depend on the size and shape of an object? Steel sinks, yet we make boats out of steel! How is this possible?  So far we have found that some objects float, and others sink  There must be a pattern to which ones float, and which sink - can you work it out?  Is it that all heavy objects sink, and all light ones float? Does it depend on the size and shape of an object? Steel sinks, yet we make boats out of steel! How is this possible?

Experiment  You have been provided with different objects, all weighing the same, but of different sizes  See if you can predict which will float, and which will sink  Write down your predictions, and your reasons for this  You have been provided with different objects, all weighing the same, but of different sizes  See if you can predict which will float, and which will sink  Write down your predictions, and your reasons for this

Results ObjectPrediction Sink / Float Result Sink / Float 10g mass 10g of rock 10g of wood 10g of wax 10g of rubber bung

Density  The reason some objects float and others sink is due to density  Density is an equation of an object’s mass divided by its volume Density = mass / volume  The reason some objects float and others sink is due to density  Density is an equation of an object’s mass divided by its volume Density = mass / volume  If an object is more dense than water, it sinks  If it is less dense, it floats  If an object is more dense than water, it sinks  If it is less dense, it floats

Steel versus Steel Boat 100kg Steel 100kg Steel Steel bar = more dense than water  sinks Steel boat = less dense than water  floats

How Do Life Jackets Work?