1. Chapter 13
Section 3. Buoyancy

2. Objectives
Explain the effect of buoyancy on the apparent weight of an object.
Explain the relationship between the volume of fluid displaced by an object and buoyant force acting on the object according to Archimedes.
Describe the relationship among density , fluid density, and whether and object sinks or floats in a fluid.
Describe the relationship among object weight, buoyant force, and whether an object sinks or floats in a fluid.

3. Vocabulary
Buoyancy
Buoyant force
Archimedes principle

4. Buoyancy Lab Please share your calculated buoyancies…
The winner will be awarded a prize.
Recap:
Did you finish the questions?
What did you learn about buoyant force?
We’ll go through this lecture and when finished maybe a few questions will have been answered.

5. Buoyant Force
Think about the observations you have made when in a swimming pool.
Is it easier or more difficult to lift stuff up?
Easier, right?
What you were experiencing is buoyancy.

6. Buoyant Force What is buoyancy?
It is the ability of a fluid to exert an upward force on an object placed in it.
What is observed is an apparent loss of weight of an object in that fluid.
& EVERY object in a fluid experiences it…to a varying degree.
It is dependent upon several factors.

7. Buoyant Force Which direction does buoyant force act?
This should be obvious.
But how can you explain this?
You must remember the principles of pressures in a fluid.
That they grow with depth, dependent upon the type of fluid, & at any given depth they are equally spread out in all directions.
Therefore, the forces increase as you go down so the force acting from the bottom will be greater…the buoyant force.
Consider the illustrations…

8. Buoyant Force

9. PRESSURE & FORCES INCREASE
Buoyant Force
PRESSURE & FORCES INCREASE

10. Buoyant Force
These forces are equal and opposite and are contributed by the fluid pressure.
FORCES INCREASE

11. Buoyant Force FORCES INCREASE
For an object in a fluid, the force opposing the buoyant force is weight.
THIS FORCE IS LARGER: THIS IS THE BUOYANT FORCE

12. Buoyant Force So why does this object sink?
Weight
FORCES INCREASE
So why does this object sink?
It has to have an unbalanced net force acting on it.

13. Archimedes Principle
This leads into Archimedes principle which allows us to determine the buoyant force.
It states that the buoyant force is equal to the weight of the fluid that is displaced by the object.
So if we put an object into a full glass of water, the weight of the water spilled out would = the buoyant force.
But this only explains the buoyant force acting on the object.
It doesn’t explain why some objects float and why others sink…completely.

14. Density & Buoyancy
Why an object floats can be dealing with density of the object compared to the density of the fluid it’s placed in.
Copy down this chart:
An object will…
Float
Sink
When the object’s density is less than the density of the fluid it’s in.
When the object’s density is greater than the density of the fluid.
Water has a density of 1 g/cm3. According to this fact, would the density of ice be higher or lower than water?

15. What is Density Who remembers the equation for density?
What is density a measure of?
How can density play a role in buoyancy when considering how the molecules are packed together?

16. Weight and the Buoyant Force

17. Weight and the Buoyant Force

18. Weight and the Buoyant Force

19. Density and Buoyancy: What do you know about the densities of each of these objects?
Compared to water, the object’s density is…
A: Lower
B: Equal
C: Higher

20. Using Density to Predict Whether Something Will Float or Sink..
Water has a density of 1g/cm3
Based upon this, which of these objects will sink, which will float?
1. Sink
2. Float
3. Float
4. Sink
5. Sink
6. Float

21. Density & Buoyancy
This accounts for why oil floats in water (oil is less dense than water).
But you can also determine if an object floats by analyzing the net forces acting on it.
The two forces acting on an object in fluid is
1) the buoyant force and
2) its weight (mass x gravity).
They work at the same time and in opposite directions.

22. Weight & Buoyancy An object will… Float (not sink) Sink
You calculate the buoyant force by measuring the weight of the fluid displaced when the object is submerged.
This can be done with devices like the spring scale.
You then compare this to the weight of the object &… (copy down)
An object will…
Float (not sink)
Sink
When the buoyant force is greater than or = to the weight of the submerged object.
When the buoyant force is smaller than the object’s submerged weight.

23. Volume Displacement
Ending Volume
Starting Volume
The weight of the displaced water exactly equals the buoyant force acting on the ball…even if it sinks.

24. Weight and Buoyancy: Compare the net force acting on each of these objects.
The buoyant force is…
A: Equal to or slightly larger than the weight
B: Equal to the weight
C: Smaller than the weight

25. Do you have to calculate buoyant forces and weights to determine whether something will float or not?
Who thinks yes?
The fact of the matter is no you don’t, all the time.
You can see this with your own eyes.
If you were planning on building a ship you would really want to know this though.
It always equals the weight of the displaced fluid.
As you saw with the boats in your lab, the larger the volume of the boat, the more weight it could have in it…therefore the greater amount of buoyant force.

26. What Were the Common Boat Designs?
Canoes
Boxes
Shallow
Deep
Rafts (circle)
What was the most buoyant boat?
The one that displaced the greatest amount of water!
The one with the most volume!

27. So Why Are Freighter Ships So Wide?

28. And Density Can Be Manipulated To Overcome Changing Depths.

29. Foil Ship Lab… What was the point of the lab yesterday?
We were trying to find the weight in which the ship would change from being buoyant, like in A, to sinking, like in B.
As long as the boat
floated the weight
always equaled the
buoyant force.
This point at which the
weight of your boat
exceeded the buoyant
force is what the
maximum buoyant
force for your ship was.
So, how do you
calculate the buoyant
force?

30. Buoyancy: Summary
Finish up your labs tonight (or with the remainder of the period).
These are due tomorrow.
Any questions?
I want everyone to take a few minutes to write 3 major things that they learned today. Write these as a journal entry after your notes.

31. Foil Lab Recap When Objects Float When objects sink
Buoyant Force = Weight
When objects sink
Buoyant Force < Weight
Weight
Buoyant force
Start
0.03N
Float
0.1N
0.5N
1.0N
1.5N
2.0N
2.5N
Sink
2.6N
Weight
Maximum Buoyant
Force
Buoyant
Force

32. Video on Buoyancy
Dr Dad…ugh, bare with me here…