1. Buoyancy
Notes

2. What is Buoyancy?
The ability to float in a fluid.

3. What is a Force?
Any push or pull
(unit: Newton or N)

4. What are the units for Buoyancy?
Buoyant force is measured in Newtons (N)

5. What is displacement? Displace =push away
The amount of water displaced by an object tells you the volume of an object
The weight of the displaced water is the buoyant force acting on the object

6. What is buoyant force?
Buoyant force is the upward force of a fluid on an object.

7. A fluid is any liquid or gas.
What is a fluid?
A fluid is any liquid or gas.

8. What are the types of buoyancy?
Positive buoyancy (floating):
Buoyant force is greater than weight so the object floats.
Neutral buoyancy:
Buoyant force is equal to weight so the object is suspended in the fluid.
Negative buoyancy (sinking):
Buoyant force is less than weight so the object sinks.

9. BF = Wair - Wwater How do you Calculate BF?
Buoyant Force= Weight of displaced fluid OR
BF = Wair - Wwater
Buoyant = Weight of Weight of object
Force object in air in water

10. Archimedes Discovered….
Ted Ed Video

11. ARCHIMEDES’ PRINCIPLE
The buoyant force acting on a submerged
object is equal to the weight of the fluid
the object displaces.

13. ARCHIMEDES’ PRINCIPLE: The WEIGHT of water displaced is equal to the BUOYANT FORCE.

15. Weight of Displaced water
Buoyant Force =
Weight of Displaced water N
When the object is submerged in water, it displaces an amount of water that weighs 3 N.
Therefore the buoyant force pushing up on the object is N
6 cm3
3 N
3 N N
6 cm3

16. Volume of object=Volume of displaced water
Buoyant Force =
Weight of object in air minus weight of object in water N
Wair = 7 N
Wwater = 4 N
Buoyant Force
= 7-4 = 3 N N
6 cm3
Volume of object=Volume of displaced water N N
6 cm3

17. Hint: compare the object’s weight in air and the buoyant force
Challenge! N
If we drop this object in the water will it sink or float? N
6 cm3
Hint: compare the object’s weight in air and the buoyant force N N
6 cm3

18. Hint: compare the object’s weight in air and the buoyant force
Challenge! N
If we drop this object in the water will it sink or float? N
6 cm3
Hint: compare the object’s weight in air and the buoyant force
SINK N N
7 N
6 cm3
3 N

19. Why do things float? Things float if they are less dense than
the fluid they are in.
Things float if they weigh less than the
buoyant force pushing up on them.
Things float if they are shaped so their
weight is spread out.

20. Changing Shape
The secret of how a ship floats is in the shape of the ship.

21. CHANGING MASS
Submarines have ballast tanks that can be opened to allow sea water to flow in.
As water is added, the submarine’s mass increases, but its volume stays the same.
Mass increases, but its volume stays the same.

23. CHANGING VOLUME
Fish have an organ called a swim bladder. This is an expandable organ. The organ can fill with oxygen therefore increasing volume and decreasing density.

24. SINK Density Air Density= 0.04 g/ml Balloon Density = 0.045 g/ml
OUTPUT
Density
Air Density= 0.04 g/ml
Balloon Density = g/ml
SINK

25. Density NOT MOVING UP OR DOWN Air Density= 0.04 g/ml
OUTPUT
Air Density= 0.04 g/ml
Balloon Density = g/ml
NOT MOVING UP OR DOWN

26. Density FLOAT Air Density= 0.04 g/ml Balloon Density = 0.035 g/ml
OUTPUT
Air Density= 0.04 g/ml
Balloon Density = g/ml
FLOAT

27. OUTPUT
A rock has a mass of 76 g and displaced 80 mL of water when it was submerged. What is the volume of the rock?
If it displaced a volume of 80mL, then the volume of the rock is 80mL!

28. OUTPUT
A volume of 200 mL of water is displaced by a ducky that weighs 10N. The weight of the displaced water is 20N. What is the buoyant force on the ducky and will it sink or float?
BF = 20N
The ducky will float because 20N (weight of the displaced fluid) is greater than 10N (weight of the duck).

29. OUTPUT
A little 27g fish displaced a volume of water that weighed 25N. The weight of the fish was 30N. What is the buoyant force on the fish and will it sink or float?
BF = 25N
The fish will sink because the weight of the fish is greater than the buoyant force.

30. OUTPUT
A person weighs 250N while swimming in the dead sea. When outside of the water they weigh 600N. What is the buoyant force acting on them? Will they sink or float?
BF = Wair – Wwater = 600 – 250 = 350N
The person will float because their weight in the water is less than the buoyant force.

31. OUTPUT
A dog jumps into a pool. The water holds him up because he only weighs 10N in water. On land, he weighs 45N. What is the buoyant force?
BF = 35N

32. Output
An object has a weight of 7N in air. The object displaces 16 mL of water that weighs 7N. What is the buoyant force? Will the object sink or float?
The weight of the displaced fluid is the buoyant force, so the BF = 7N. The object will be neutrally buoyant and float.