UNIT THREE: Matter, Energy, and Earth

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UNIT THREE Matter, Energy, and Earth

• Chapter 8 Matter and Temperature
• Chapter 9 Heat
• Chapter 10 Properties of Matter
• Chapter 11 Earths Atmosphere and Weather

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Buoyancy How could we explain the relationship
between density and buoyancy?
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Chapter Ten Properties of Matter

• 10.1 Density
• 10.2 Properties of Solids
• 10.3 Properties of Fluids
• 10.4 Buoyancy

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Learning Goals

• Define buoyancy.
• Explain the relationship between density and
  buoyancy.
• Discuss applications of Archimedes principle.

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Buoyancy is a force to explain why some objects
floats on water.

• Buoyancy is a measure of the upward force a fluid
  exerts on an object that is submerged.

The water in the pool exerts an upward force that
acts in a direction opposite to the boys weight.
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10.4 Volume and buoyancy

• The strength of the buoyant force on an object in
  water depends on the volume of the object that is
  underwater.

As you keep pushing downward on the ball, the
buoyant force gets stronger and stronger. -Which
ball has more volume underwater? (the 2nd ball)
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10.4 Archimedes Principal

• In the third century BC, a Greek mathematician
  named Archimedes realized that buoyant force is
  equal to the weight of liquids that is taken up
  by an object.
• A simple experiment can be done to measure the
  buoyant force on a rock with a spring scale when
  the rock is in water.

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10.4 Weight and buoyancy

• Weight is a force, like any other pushing or
  pulling force, and is caused by Earths gravity.
• It is easy to confuse mass and weight, but they
  are not the same.
• Weight is the downward force of gravity acting on
  mass.

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10.4 Weight and buoyancy

• Mass is not affected by Earths gravity, and is
  the amount of matter that the object has.
• We use Newtons to measure Weight (Force) in
  Science.
• The newton (symbol N) is the International
  System of Units (SI) derived unit of force.
• 1kg 9.8 N

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10.4 Weight and buoyancy

• 1kg 9.8 N
• What is the rocks mass?
• Mass is 3kg.
• What is the rocks weight?
• Weight is 29. 4 N (3 X 9.8kg)

What is the rocks mass? What is the rocks
weight?
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10.4 Sinking and floating

• In air the buoyant force on the rock is 29.4 N.

• When the rock was submerged, the scale read 19.6
  N.
• The difference (subtract 29.4- 19.6) is a force
  of 9.8 N, exactly the amount of force the
  displaced water exerts.

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Example Sinking and floating

• If in air the buoyant force on the plastic cube
  is 9.7 N.

• When the plastic cube was submerged, the scale
  read 2.5 N.
• What is the amount of force the displaced water
  exerts or buoyancy does the plastic cube have?
• 5.2 N will be the buoyancy of the plastic cube.
  (9.7 2.5 5.2)

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10.4 Sinking and floating
These blocks are the same total volume. Which
block has more buoyant force acting on it? Foam
Block because buoyant force pushed it higher in
the air.
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10.4 Sinking and floating
These blocks are the same total volume. Which
block weighs more in air? Foam Block because
there is less of the foam block in the water (to
cause it to float easier)
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10.4 Sinking and floating

• Buoyancy explains why some objects sink and
  others float.

• Whether an object sinks or floats depends on how
  the buoyant force compares with the weight.

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10.4 Density and buoyancy

• If you know an objects density you can quickly
  predict whether it will sink or float.

Which ball will sink in water? Which ball will
float in water?
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10.4 Density and buoyancy

• If you know an objects density you can quickly
  predict whether it will sink or float.

• Which ball will sink in water? Steel Ball, the
  density is higher, 7.8 is higher than 1
• Which ball will float in water? Wood Ball, the
  density is lower, .75 is lower than 1

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10.4 Density and buoyancy

• When they are completely underwater, both balls
  have the same buoyant force because they displace
  the same volume of water.

• However, the steel ball has more weight since it
  has a higher density.

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10.4 Boats and apparent density

• Apparent density determines whether an object
  sinks or floats.

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10.4 Apparent Density

• An object with an apparent density GREATER than
  the density of water will sink.
• An object with an apparent density LESS than the
  density of water will float.

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10.4 Buoyancy, volume, temperature, and pressure
of gases

• A hot-air balloon floats because the air inside
  is less dense than the air outside.
• The balloon example illustrates an important
  relationship, known as Charless law, discovered
  by Jacques Charles in 1787.

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10.4 Charles Law

• According to Charles law, the volume of a gas
  increases with increasing temperature.
• Volume decreases with decreasing temperature.

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10.4 Pressure-Temperature Relationship

• The pressure of a gas is also affected by
  temperature changes.
• If the mass and volume are kept constant, the
  pressure goes up when the temperature goes up,
  and the pressure goes down when the temperature
  goes down.

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10.4 Pressure-Temperature Relationship

• The mathematical relationship between the
  temperature and pressure of a gas at constant
  volume and mass was discovered by Joseph
  Gay-Lussac in 1802.

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10.4 Converting to Kelvin

• Any time you see a temperature in a formula in
  this section about gases, the temperature must be
  in Kelvins.

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Solving Problems

• A can of hair spray has a pressure of 300 psi at
  room temperature 21C.
• The can is accidentally moved too close to a fire
  and its temperature increases to 295C.
• What is the final pressure in the can? (Round
  answer to nearest whole number.)

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Solving Problems

• Looking for
• final pressure in psi(P2)
• Given
• P1 300 psi, T1 21 ?C , T2 295 ?C
• Relationships
• Convert temps using K ?C 273
• Charles Law P1/T1 P2/T2
• Solution
• Rearrange equation so P2 P1xT2 / T1
• P2 300 psi. x 568K / 294K 580 psi.

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Investigation 10C
Mountains and Earths Crust

• Key Question
• How can mountains float?

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Density and Ocean Currents

• Did you know that there are underwater waterfalls
  in the ocean?
• While it may seem strange for water to fall
  through water, it really happens due to density
  differences in ocean water coming from different
  sources.