Density and Buoyancy. Density Which would have more mass? It depends on BOTH the size of the object AND the material contained inside

Density and BuoyancyDensity and Buoyancy

DensityDensity

Which would have more mass?Which would have more mass?

It depends on BOTH the size of the object It depends on BOTH the size of the object AND the material contained inside.AND the material contained inside.

DensityDensity

Density is the amount of mass per unit Density is the amount of mass per unit volume of a material.volume of a material.

Matter is made of atoms which are Matter is made of atoms which are particles that have mass.particles that have mass.

Density depends on mass AND the Density depends on mass AND the number of particles packed into a given number of particles packed into a given volume.volume.

DensityDensity

DensityDensity

The volume of air has fewer particles, thus The volume of air has fewer particles, thus less mass than the less mass than the samesame volume of water. volume of water.

Thus, the density of air is Thus, the density of air is lessless than the than the density of water.density of water.

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DensityDensity

D = m

VD – density, the unit is g / cm3 (solids) or

g / mL (liquids)m – mass, the unit is gV – volume, the unit is cm3 (solids) or

mL (liquids)

DensityDensity

Density depends on the material an object Density depends on the material an object is made from, NOT its size.is made from, NOT its size.

An entire chocolate bar will have the same An entire chocolate bar will have the same density as just one piece. It’s all made density as just one piece. It’s all made from the same material – chocolate.from the same material – chocolate.

DensityDensity

Density depends on the mass of the Density depends on the mass of the particles. (more mass = more density)particles. (more mass = more density)

Density depends on the distance between Density depends on the distance between particles. (more distance = less density)particles. (more distance = less density)

Gases are usually less dense than solids Gases are usually less dense than solids or liquids because particles are far apart.or liquids because particles are far apart.

DensityDensity

DensityDensity

DensityDensity

To make something less dense, you can To make something less dense, you can decreasedecrease the the massmass oror increaseincrease the the volumevolume of an object.of an object.

A solid steel ball sinks in water, but that A solid steel ball sinks in water, but that same mass of steel made into a curved same mass of steel made into a curved hull of a boat will float – the shape of the hull of a boat will float – the shape of the hull causes it to displace a greater volume hull causes it to displace a greater volume of water than a solid ball.of water than a solid ball.

Measuring DensityMeasuring Density

1.1. Measure its mass.Measure its mass.

2.2. Measure its volume.Measure its volume.

3.3. Divide its mass by its volume.Divide its mass by its volume.

Density is a Physical PropertyDensity is a Physical Property

A physical property is something you can A physical property is something you can measure without changing the composition measure without changing the composition of the material.of the material.

When you measure an object’s mass and When you measure an object’s mass and volume, you don’t change it into something volume, you don’t change it into something else.else.

Pressure in a FluidPressure in a Fluid

A fluid is a material that can flow and has A fluid is a material that can flow and has no definite shape.no definite shape.

Liquids and gases are fluids.Liquids and gases are fluids. Fluids can exert forces.Fluids can exert forces.


Pressure is the force Pressure is the force per unit of area per unit of area applied on the applied on the surface of an object.surface of an object.


Pressure depends Pressure depends on the area over on the area over which a force is which a force is exerted.exerted.

If the area increases, If the area increases, pressure decreases.pressure decreases.


Pressure depends on the size of the force.Pressure depends on the size of the force. If the force increases, pressure increases.If the force increases, pressure increases.


The equation for pressure is: P = F ÷ AThe equation for pressure is: P = F ÷ A

Pressure = force (newtons)Pressure = force (newtons)

area (meters squared)area (meters squared)


The SI unit of pressure is the pascal, The SI unit of pressure is the pascal, abbreviated Pa. A pressure of 1 Pa is abbreviated Pa. A pressure of 1 Pa is equal to a force of 1 N applied over an equal to a force of 1 N applied over an area of 1 marea of 1 m22

1 Pa = 1 N / m1 Pa = 1 N / m22


For any liquid in a For any liquid in a container of any shape, the container of any shape, the pressure depends only on pressure depends only on the height of the column of the height of the column of liquid liquid aboveabove the surface the surface where you measure the where you measure the pressure.pressure.


The deeper The deeper underwater you go, underwater you go, the greater the the greater the pressure is on you. pressure is on you. This is because as This is because as you go deeper, the you go deeper, the column of water column of water above you gets above you gets taller.taller.

Pressure in a FluidPressure in a Fluid On April 10, 1963, the USS Thresher On April 10, 1963, the USS Thresher

nuclear-powered attack submarine was nuclear-powered attack submarine was lost at sea during deep-diving tests. All 129 lost at sea during deep-diving tests. All 129 aboard died. (8400 ft / 2600 m below sea aboard died. (8400 ft / 2600 m below sea level).level).


In a fluid, pressure is exerted on all points In a fluid, pressure is exerted on all points on the outside of an object in a direction on the outside of an object in a direction perpendicularperpendicular to the surface of the object to the surface of the object at that point.at that point.


A gas, like any fluid, exerts a pressure on A gas, like any fluid, exerts a pressure on an object. That pressure depends on the an object. That pressure depends on the height of the gas above the object.height of the gas above the object.


Atmospheric pressure is the force exerted Atmospheric pressure is the force exerted per unit area by air particles.per unit area by air particles.

As you climb, or increase your elevation, As you climb, or increase your elevation, atmospheric pressure atmospheric pressure decreasesdecreases..

mb = millibar (100 Pa)


Atmospheric pressure Atmospheric pressure at sea level is caused at sea level is caused by a large weight of by a large weight of gas, but is difficult to gas, but is difficult to notice because there notice because there is an equal, internal is an equal, internal pressure pushing out pressure pushing out from the inside of your from the inside of your body.body.

Buoyant ForceBuoyant Force

The buoyant force is The buoyant force is the the netnet upward force upward force exerted on an object in exerted on an object in a fluid by the a fluid by the surrounding fluid. It surrounding fluid. It acts against gravity.acts against gravity.

Buoyant force is the Buoyant force is the DIFFERENCE between DIFFERENCE between the pressure above and the pressure above and the pressure below.the pressure below.


In a fluid, pressure increases with depth.In a fluid, pressure increases with depth. The force of the fluid pushing The force of the fluid pushing upup on the on the

bottom of an object suspended in water is bottom of an object suspended in water is always always greatergreater than the force of the fluid than the force of the fluid pushing down on the top of the same pushing down on the top of the same object.object.


The size of the buoyant force does The size of the buoyant force does NOTNOT depend on depth.depend on depth.


The buoyant force does NOT change as The buoyant force does NOT change as an object goes deeper because it’s a an object goes deeper because it’s a DIFFERENCE between the pressure DIFFERENCE between the pressure above and the pressure below. Both those above and the pressure below. Both those increase so the buoyant force stays the increase so the buoyant force stays the same. same.


5 N

10 N10 N

15 N


At the fish goes deeper, the force on its back increases.

However, the force on its belly also increases to match and keep the fish suspended in the water.

The net buoyant force is always the same, no matter what the depth.

Buoyant ForceBuoyant Force Archimedes’ Principle states that the Archimedes’ Principle states that the

buoyant force on an object is equal to the buoyant force on an object is equal to the weightweight of the fluid the object displaces. of the fluid the object displaces.

This is why objectsfeel lighter in water!

Why do objects sink or float?

Why do objects sink or float?Why do objects sink or float?

An object will float if the density of the An object will float if the density of the object is LESS than the density of the fluid.object is 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 Gravity and the buoyant force act on the object in opposite directions.object in opposite directions.


A submerged object displaces a volume of A submerged object displaces a volume of fluid equal to its own volume.fluid 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 equal to the volume of the portion underwater.underwater.


If the upward buoyant force If the upward buoyant force is less than the object’s is less than the object’s weight, the net force on the weight, the net force on the object is downward.object is 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?Why do objects sink or float?Archimedes’ Principle…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 observed that an object immersed in water sinks or floats depending on the water sinks or floats depending on the WEIGHT OF THE WATER it displaces.WEIGHT OF THE WATER it displaces.


Negative Buoyancy – Negative Buoyancy – (sinking) when the (sinking) when the object weighs MORE object weighs MORE than the weight of the than the weight of the water displacedwater displaced


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 you put an object) when the objects weighs the SAME as the weight of the weighs the SAME as the weight of the waterwater

The Buoyant Force & DensityThe 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.

The Buoyant Force & DensityThe Buoyant Force & Density Almost all metals have a density greater Almost all metals have a density greater

than water (1 g/mL), so they sink.than water (1 g/mL), so they sink.

The Buoyant Force & DensityThe Buoyant Force & Density BUT, the density of a metal ship BUT, the density of a metal ship

is LESS than water because a is LESS than water because a large volume of the ship is filled large volume of the ship is filled with air.with air.

Floating & Sinking in the Floating & Sinking in the AtmosphereAtmosphere

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

An object will float in air only if the buoyant An object will float in air only if the buoyant force from air pressure on an object is force from air pressure on an object is large enough.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 with helium, it is less dense than the air around dense than the air around it, and so the balloon it, and so the balloon floats.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 be controlled by changing the density of the air inside the balloon.the air inside the balloon.


A burner heats the air in the hot-air A burner heats the air in the hot-air balloon.balloon.

Heat makes the air molecules move Heat makes the air molecules move farther apart from each other.farther apart from each other.

The hot air inside becomes less dense The hot air inside becomes less dense than the air outside the balloon, and it than the air outside the balloon, and it floats.floats.


To bring the balloon back down, cool the To bring the balloon back down, cool the air.air.

The air particles inside come closer The air particles inside come closer together, and the balloon’s total density is together, and the balloon’s total density is more than the air outside, so it sinks. more than the air outside, so it sinks.

http://www.youtube.com/watch?http://www.youtube.com/watch?v=oCkacB_72i0v=oCkacB_72i0

http://www.youtube.com/watch?v=oCkacB_72i0&feature=results_video&playnext=1&list=PL3AEDD740377D6186

REVIEWREVIEW

1. How does the density of an object 1. How does the density of an object floating on a fluid compare to the density floating on a fluid compare to the density of the fluid?of the fluid? A. The floating object is less dense.A. The floating object is less dense. B. The floating object is equally dense.B. The floating object is equally dense. C. The floating object is denser.C. The floating object is denser. D. The density cannot be compared.D. The density cannot be compared.

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2. The force keeping an object floating on 2. The force keeping an object floating on a fluid is called a fluid is called A. ViscosityA. Viscosity B. Buoyant forceB. Buoyant force C. Natural forceC. Natural force D. Surface tensionD. Surface tension

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3. Pressure is the _____ per unit area.3. Pressure is the _____ per unit area. A. massA. mass B. volumeB. volume C. forceC. force D. pascalD. pascal

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4. Why does a hot air balloon float?4. Why does a hot air balloon float? A. The surrounding air is denser than the A. The surrounding air is denser than the

balloon.balloon. B. The surrounding air is less dense than the B. The surrounding air is less dense than the

balloon.balloon. C. The balloon is denser than the surrounding C. The balloon is denser than the surrounding

air.air. D. The balloon has less mass than the D. The balloon has less mass than the

surrounding air. surrounding air.

REVIEWREVIEW

5. An object that is _____ than water will 5. An object that is _____ than water will _______._______. A. less dense; sinkA. less dense; sink B. denser; floatB. denser; float C. less dense; lose massC. less dense; lose mass D. denser; sinkD. denser; sink

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Density and Buoyancy. Density Which would have more mass? It depends on BOTH the size of the object AND the material contained inside