3-3 notes – sinking & floating chapter 3, lesson 3

3-3 Notes – Sinking & FloatingChapter 3, Lesson 3Chapter 3, Lesson 3

You Can CompareDensity...

Why do objects sink or float?

Why do objects sink or float?• An object will float if the density of the object is An object will float if the density of the object is

LESS than the density of the fluid.LESS than the density of the fluid.

• Fluids exert pressure on any object in Fluids exert pressure on any object in the fluid.the fluid.

• Gravity and the buoyant force act on the object Gravity and the buoyant force act on the object in opposite directions.in opposite directions.

You Can CompareVolume...


Why do objects sink or float?• A submerged object displaces a volume of fluid A submerged object displaces a volume of fluid

equal to its own volume.equal to its own volume.

• A floating object displaces a volume of fluid A floating object displaces a volume of fluid equal to the volume of the portion underwater.equal to the volume of the portion underwater.

You Can Compare Weight…


Why do objects sink or float?• If the upward buoyant force is If the upward buoyant force is

less than the object’s weight, less than the object’s weight, the net force on the object is the net force on the object is downward.downward.

• The object accelerates The object accelerates downward because of the downward because of the unbalanced force.unbalanced force.

Why do objects sink or float?• When an object in a When an object in a

fluid isn’t accelerating, fluid isn’t accelerating, the forces are balanced.the forces are balanced.

• The weight of the The weight of the woman is balanced by woman is balanced by the buoyant force the buoyant force pushing upward on the pushing upward on the woman.woman. buoyant forcegravity

Why do objects sink or float?Archimedes’ Principle…

• Archimedes’ (287 B.C.E. to 212 B.C.E.) Archimedes’ (287 B.C.E. to 212 B.C.E.) observed that an object immersed in water observed that an object immersed in water sinks or floats depending on the WEIGHT sinks or floats depending on the WEIGHT OF THE WATER it displaces.OF THE WATER it displaces.

So the story goes...Before the Greek mathematician and inventor Before the Greek mathematician and inventor Archimedes (C.287-212 B.C.), nobody knew why Archimedes (C.287-212 B.C.), nobody knew why some things float and others don’t. It is said that some things float and others don’t. It is said that Archimedes’ king gave a silversmith a certain Archimedes’ king gave a silversmith a certain amount of gold to make him a new crown. When the amount of gold to make him a new crown. When the crown was finished, the king asked Archimedes to crown was finished, the king asked Archimedes to figure out a way to secretly determine if the crown figure out a way to secretly determine if the crown was made of pure gold or a mix of gold and cheaper was made of pure gold or a mix of gold and cheaper metals, as the king suspected.metals, as the king suspected.

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

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

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


• Negative Buoyancy – Negative Buoyancy – (sinking) when the object (sinking) when the object weighs MORE than the weighs MORE than the weight of the water weight of the water displaceddisplaced


• Positive Buoyancy – (floating) when the Positive Buoyancy – (floating) when the object weighs LESS than the weight of the object weighs LESS than the weight of the water displaced.water displaced.


• Neutral Buoyancy – (hovering wherever Neutral Buoyancy – (hovering wherever you put an object) when the objects weighs you put an object) when the objects weighs the SAME as the weight of the waterthe SAME as the weight of the water

The Buoyant Force & Density

• An object floats if it is LESS dense than the An object floats if it is LESS dense than the fluid around it.fluid around it.


• Almost all metals have a density greater than Almost all metals have a density greater than water (1 g/mL), so they sink.water (1 g/mL), so they sink.


• BUT, the density of a metal ship is BUT, the density of a metal ship is LESS than water because a large LESS than water because a large volume of the ship is filled with air.volume of the ship is filled with air.


• A hydrometer is an A hydrometer is an instrument that measures the instrument that measures the density of a fluid.density of a fluid.

The Buoyant Force & Density• In a less dense fluid, a hydrometer floats In a less dense fluid, a hydrometer floats

LOWER because it displaces more fluid.LOWER because it displaces more fluid.

The Buoyant Force & Density• In a more dense fluid, a hydrometer floats In a more dense fluid, a hydrometer floats

HIGHER because it displaces less fluid.HIGHER because it displaces less fluid.


Dense Liquidsea water

(saltwater)1025 kg/m3

Liquidfresh water (lake water)1000 kg/m3

Not Dense Liquidoil

800 kg/m3

Floating & Sinking in the Atmosphere

• Air is a fluid and it exerts pressure on things Air is a fluid and it exerts pressure on things around it.around it.

• An object will float in air only if the An object will float in air only if the buoyant force from air pressure on an object buoyant force from air pressure on an object is large enough.is large enough.


• Helium gas is less dense Helium gas is less dense than nitrogen gas and than nitrogen gas and oxygen gas, which are the oxygen gas, which are the main components of air.main components of air.

• When a balloon is filled When a balloon is filled with helium, it is less dense with helium, it is less dense than the air around it, and so than the air around it, and so the balloon floats.the balloon floats.


• Whether a hot-air balloon rises or falls can Whether a hot-air balloon rises or falls can be controlled by changing the density of the be controlled by changing the density of the air inside the balloon.air inside the balloon.


• A burner heats the air in the hot-air balloon.A burner heats the air in the hot-air balloon.• Heat makes the air molecules move farther Heat makes the air molecules move farther

apart from each other.apart from each other.• The hot air inside becomes less dense than The hot air inside becomes less dense than

the air outside the balloon, and it floats.the air outside the balloon, and it floats.

Floating & Sinking in the Atmosphere• To bring the balloon back down, cool the air.To bring the balloon back down, cool the air.• The air particles inside come closer together, The air particles inside come closer together,

and the balloon’s total density is more than and the balloon’s total density is more than the air outside, so it sinks. the air outside, so it sinks.

Invisible water? Super dense gas?

Do NOT try this at home!!! Ever. Rly.http://www.youtube.com/watch?v=oCkacB_72i0

1:30

http://www.youtube.com/watch?v=oCkacB_72i0

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

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.


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.


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

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

SCI 8.d