1. 3-3 Notes – Sinking & Floating
Chapter 3, Lesson 3

2. You Can Compare Density...
Why do objects sink or float?
You Can Compare Density...

3. Why do objects sink or float?
An object will float if the density of the object is LESS than the density of the fluid.
Fluids exert pressure on any object in the fluid.
Gravity and the buoyant force act on the object in opposite directions.

4. Why do objects sink or float?
You Can Compare Volume...

5. Why do objects sink or float?
A submerged object displaces a volume of fluid equal to its own volume.
A floating object displaces a volume of fluid equal to the volume of the portion underwater.

6. You Can Compare Weight…
Why do objects sink or float?
You Can Compare Weight…

7. Why do objects sink or float?
If the upward buoyant force is less than the object’s weight, the net force on the object is downward.
The object accelerates downward because of the unbalanced force.

8. Why do objects sink or float?
When an object in a fluid isn’t accelerating, the forces are balanced.
The weight of the woman is balanced by the buoyant force pushing upward on the woman.
gravity
buoyant force

9. Why do objects sink or float? Archimedes’ Principle…
Archimedes’ (287 B.C.E. to 212 B.C.E.) observed that an object immersed in water sinks or floats depending on the WEIGHT OF THE WATER it displaces.

10. So the story goes...
Before the Greek mathematician and inventor Archimedes (C B.C.), nobody knew why some things float and others don’t. It is said that Archimedes’ king gave a silversmith a certain amount of gold to make him a new crown. When the crown was finished, the king asked Archimedes to figure out a way to secretly determine if the crown was made of pure gold or a mix of gold and cheaper metals, as the king suspected.

11. So the story goes...
Archimedes was puzzled as to how to do this, until he stepped into a bathtub and observed that his body displaced (pushed out a place) a certain volume (amount of space a material occupies) of water from the tub. Gold has a specific density, or mass per unit volume. This means that there is a specific amount of gold material in a given volume.

12. So the story goes...
Archimedes realized that he could determine the volume of the crown by measuring the volume of water displaced by the crown, and using the volume and the mass of the crown, he could determine its density. It is said that he was so excited about his discovery that he ran naked through the street yelling, “Eureka!” (I have found it!).

13. So the story goes...
He determined that the density of the crown was not that of pure gold, and the silversmith who made the crown was killed for stealing from the king. Whether this story is true or not, Archimedes did discover a technique for determining density using water displacement.

14. Why do objects sink or float?
Negative Buoyancy – (sinking) when the object weighs MORE than the weight of the water displaced

15. Why do objects sink or float?
Positive Buoyancy – (floating) when the object weighs LESS than the weight of the water displaced.

16. Why do objects sink or float?
Neutral Buoyancy – (hovering wherever you put an object) when the objects weighs the SAME as the weight of the water

17. The Buoyant Force & Density
An object floats if it is LESS dense than the fluid around it.

18. The Buoyant Force & Density
Almost all metals have a density greater than water (1 g/mL), so they sink.

19. The Buoyant Force & Density
BUT, the density of a metal ship is LESS than water because a large volume of the ship is filled with air.

20. The Buoyant Force & Density
A hydrometer is an instrument that measures the density of a fluid.

21. The Buoyant Force & Density
In a less dense fluid, a hydrometer floats LOWER because it displaces more fluid.

22. The Buoyant Force & Density
In a more dense fluid, a hydrometer floats HIGHER because it displaces less fluid.

23. The Buoyant Force & Density

24. The Buoyant Force & Density
Dense Liquid
sea water (saltwater)
1025 kg/m3
Liquid
fresh water
(lake water)
1000 kg/m3
Not Dense Liquid
oil
800 kg/m3

25. Floating & Sinking in the Atmosphere
Air is a fluid and it exerts pressure on things around it.
An object will float in air only if the buoyant force from air pressure on an object is large enough.

26. Floating & Sinking in the Atmosphere
Helium gas is less dense than nitrogen gas and oxygen gas, which are the main components of air.
When a balloon is filled with helium, it is less dense than the air around it, and so the balloon floats.

27. Floating & Sinking in the Atmosphere
Whether a hot-air balloon rises or falls can be controlled by changing the density of the air inside the balloon.

28. Floating & Sinking in the Atmosphere
A burner heats the air in the hot-air balloon.
Heat makes the air molecules move farther apart from each other.
The hot air inside becomes less dense than the air outside the balloon, and it floats.

29. Floating & Sinking in the Atmosphere
To bring the balloon back down, cool the air.
The air particles inside come closer together, and the balloon’s total density is more than the air outside, so it sinks.

30. Invisible water? Super dense gas?
Do NOT try this at home!!! Ever. Rly.
1:30

31. What does a hydrometer measure? A mass of a solid B density of a solid
3.3 Sinking and Floating
What does a hydrometer measure?
A mass of a solid
B density of a solid
C mass of a liquid
D density of a liquid

32. Why does a hot-air balloon float?
3.3 Sinking and Floating
Why does a hot-air balloon float?
A The surrounding air is denser than the balloon.
B The surrounding air is less dense than the balloon.
C The balloon is denser than the surrounding air.
D The balloon has less mass than the surrounding air.

33. Why does an object sink in water? A The object is denser than water.
3.3 Sinking and Floating
Why does an object sink in water?
A The object is denser than water.
B The buoyant force is greater than the object’s weight.
C The buoyant force is greater than the object’s mass.
D The object is less dense than water.

34. SCI 8.d
An object with a mass of 9 g displaces 3 mL of water. It is placed in a fluid with a density of 2 g/cm3. The object ____ in the fluid.
A floats
B sinks
C loses mass
D becomes denser

35. An object that is ____ than water will ____. A less dense; sink
SCI 8.d
An object that is ____ than water will ____.
A less dense; sink
B denser; float
C less dense; lose mass
D denser; sink