Dr. McDowell IPC 01/23/04 B day

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Dr. McDowell IPC 01/23/04 B day

• Todays Warm Up
• Questions to ponder
• If I drop a steel bolt into a glass of water, it
  would sink, right?
• Then, why doesnt a steel boat sink?
• What makes a steel boat float?
• TEST over Chapters 16 17NEXT TIME

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Todays Agenda

• Wrap Up Chapter 17 Notes
• Buoyancy (with an activity)
• Viscosity
• Properties of Gases
• Chapter 17 Recap
• Chapter 16-17 Practice Test

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CH 17.3 Notes

• Have you ever noticed how its easy to lift
  things in a swimming pool?
• Whats helping you lift?

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Write this in your Notes

• Water exerts an upward force on floating objects.
• This upward force is called buoyancy.

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Write this in your Notes

• For something to float, the water must provide an
  upward buoyant force to overcome the downward
  force of gravity.

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• What do we call the downward force due to
  gravity?
• --- Weight

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Write this in your Notes

• For an object to float, the buoyant force must
  equal the weight of the object.

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Write this in your Notes

• Archimedes Principle says that the buoyant force
  on an object is equal to the weight of the water
  displaced by the object.
• So, the more water displaced, the larger the
  buoyant force!

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• So how can we make an object displace more water?
• By changing its shape!

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Buoyancy Activity

• The instructions are simple
• Make the hunk of clay float in water
• IMPORTANT When testing, remove your clay from
  the water as quickly as possible. If it gets too
  wet, it will get slimy!

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Buoyancy Take Home Message

• An object only floats when the buoyant force from
  the liquid is equal to or greater than the weight
  of the object.
• The buoyant force is determined by the weight of
  the displaced water.
• The hunk of clay didnt displace enough water.
• Because the clay boat displaced more water, it
  floated!

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Difference in Liquids

• What is the difference in pouring pancake syrup
  and water?
• Why is it harder to pour the pancake syrup?
• How were these liquids different?

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Write this in your Notes

• Viscosity is a property of fluids that measures
  the materials resistance to flow.
• Viscous (thick) liquids are made of large, bumpy
  molecules that create friction as the slide past
  one another.
• As a liquid gets warmer, its viscosity decreases,
  meaning it flows easier.

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Do you remember our discussionfrom last time?

• Why do hot air balloons float?
• It had something to do with hot air right?

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• Because the heated air molecules are moving with
  lots of kinetic energy, they become more
  separated making them less dense than cool air.
• Being less dense than air, the hot air in the
  balloon rises.

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• If we want to continue to talk about gases, we
  will have to talk about pressure.
• What is pressure?

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Write this in your Notes

• Pressure is a force acting on an area of a
  surface
• Pressure has several units
• Pounds per square inch (psi)
• Pascal (Pa) is the scientific unit for pressure
• Others units are atm and bar

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• What is atmospheric pressure?

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• Atmospheric pressure comes from the air in our
  atmosphere.
• Have you noticed the weatherman report
  atmospheric pressure? What kind of number is
  reported?

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• Normal Atmospheric Pressure ranges from 28 to 30
  inches of Mercury (yet another unit for pressure)
• Thats a little over 100,000 Pascals, or 100,000
  N per m2 (about the weight of an elephant!)
• At higher altitudes water boils at a lower
  temperature

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• How are pressure and volume related?
• How are pressure and temperature related?

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Write this in your Notes

• Boyles Law states that the pressure of a gas
  increases when its volume decreases.
• Guy-Lussacs Law states the pressure of a gas
  increases when its temperature increases.
• Does this make sense?

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Consider this

• Suppose I pump equal amounts of air into a soccer
  ball and a basketball. Which ball has the higher
  pressure?
• Because the soccer ball has a smaller volume,
  its pressure will be higher.
• This is Boyles Law, decreasing the volume of the
  ball increased the pressure!

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Consider this

• After leaving your basketball outside overnight,
  you find that its doesnt bounce very well the
  next morning. Can you explain what happened?
• Because the basketball was cold, the pressure
  inside decreased.
• This is Guy-Lussacs Law, decreasing temperature
  decreases pressure