1. Why Things Float
D. Crowley, 2007

2. Why Things Float To understand why objects float
Wednesday, April 19, 2017
Why Things Float
To understand why objects float

3. 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)?

4. Drawing
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)?
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!
Gravity

5. Floaters 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…
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…
Gravity

6. 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?!

7. Experiment
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

8. Results Object Prediction Sink / Float Result 10g mass 10g of rock
10g of wood
10g of wax
10g of rubber bung

9. 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
If an object is more dense than water, it sinks
If it is less dense, it floats!

10. Floating 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

11. Floating
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!

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

13. Floating
Complete the floating worksheet

14. Floating Water Upthrust Weight Gravity 1600N
You would need a minimum upthrust force of 1600N!