Why Things FloatWhy Things Float

D. Crowley, 2007D. Crowley, 2007

Why Things FloatWhy Things Float

To understand why objects floatTo understand why objects float

Wednesday, April 19, 2023

FloatingFloating 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)?

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)?

DrawingDrawing In your own words (including adding a diagram of a

boat) explain why the boat stays afloat. Include force arrows, as well as what forces they are (what is pulling the boat down, what is reacting against this force)?

In your own words (including adding a diagram of a boat) explain why the boat stays afloat. Include force arrows, as well as what forces they are (what is pulling the boat down, what is reacting against this force)?

Gravity

Reaction Force - up thrust of water

Both arrows are the same size - meaning the boat remains where it is (i.e. it doesn’t sink or lift out of the water)

If the force down was greater he boat would sink; if the force up was greater the boat would rise out from the water!

FloatersFloaters

So why is it some objects float, and others sink?

Look at the picture below of some rocks sinking. Copy this, and draw your own force labels, explaining what is going on…

So why is it some objects float, and others sink?

Look at the picture below of some rocks sinking. Copy this, and draw your own force labels, explaining what is going on…

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…

ExperimentExperiment

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?!

ExperimentExperiment

You have been provided 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 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

ResultsResults

Object PredictionSink / Float

ResultSink / Float

10g mass

10g of rock

10g of wood

10g of wax

10g of rubber bung

DensityDensity 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!

FloatingFloating

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?

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

FloatingFloating Objects will either sink of float, depending upon their

density

If they are more dense than water, they sink. Less dense, and they float

The shape of an object has a lot to do whether it sinks or not - 100kg of steel will sink, but 100kg of steel shaped into a boat will float, because overall the volume of the boat is much bigger (it contains a great deal of space which isn’t steel), so its overall density is reduced

*Buoyancy defined: an object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object – if the boat can displace a greater mass of liquid than its own mass, then it will float!

Objects will either sink of float, depending upon their density

If they are more dense than water, they sink. Less dense, and they float

The shape of an object has a lot to do whether it sinks or not - 100kg of steel will sink, but 100kg of steel shaped into a boat will float, because overall the volume of the boat is much bigger (it contains a great deal of space which isn’t steel), so its overall density is reduced

*Buoyancy defined: an object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object – if the boat can displace a greater mass of liquid than its own mass, then it will float!

Steel versus Steel BoatSteel versus Steel Boat

100kg Steel

100kgSteel

Steel bar = more dense than water sinks

Steel boat = less dense than water floats

FloatingFloating

Complete the floating worksheet

Complete the floating worksheet

FloatingFloatingWaterUpthrust

WeightUpthrust

GravityUpthrust

UpthrustGravity

1600N

You would need a minimum upthrust force of 1600N!

WaterUpthrust

WeightUpthrust

GravityUpthrust

UpthrustGravity

1600N

You would need a minimum upthrust force of 1600N!