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PHYSICS · Stage 9 · Chapter 11: Forces and Density

Physics 11.3 Chapter 11 Review: Forces and Density

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Physics 11.3 Chapter 11 Review: Forces and Density — Alhussan International Academy — Physics (Stages 7–9) notes: Summarise Chapter 11 on density, floating and sinking, upthrust, the swim bladder and the Galileo thermometer.Equation — Density = mass ÷ volume, in g/cm³ or kg/m³.Re…

  • Rearranged
  • Irregular objects
  • Comparing with water
  • Different liquids
  • Average density
  • Staying at one depth

Section 1 of 4Density

  • Equation
    • Density = mass ÷ volume, in g/cm³ or kg/m³.
  • Rearranged
    • Mass = density × volume, and volume = mass ÷ density.
  • Irregular objects
    • Find the volume by displacement: volume = final reading − starting reading in a measuring cylinder.
  • Comparing with water
    • Water has a density of 1.0 g/cm³, so denser objects sink and less dense objects float.

Section 2 of 4Floating, sinking and upthrust

  • Floating
    • An object floats if its density is less than the liquid's, or if the upthrust equals its weight.
  • Upthrust
    • It exists because the pressure at the bottom of an object is higher than at the top.
  • Different liquids
    • Cooking oil (0.9 g/cm³) floats on water, while glycerine (1.26 g/cm³) sinks below it.
  • Average density
    • Objects with air spaces, such as ships, have a lower average density than the material they are made from.

Section 3 of 4The swim bladder of a fish

  • Swim bladder = a sac inside a fish that it can fill with gas.
  • Rising
    • Filling the sac with oxygen from the gills increases the fish's volume without adding much mass,
    • so its average density falls below the water's and upthrust exceeds weight.
  • Sinking
    • Removing gas raises its average density, so it goes down.
  • Staying at one depth
    • The fish's density equals the water's density and upthrust equals weight.

Section 4 of 4The Galileo thermometer

  • Design
    • A sealed glass tube of liquid contains sealed glass bulbs, each with a metal tag showing a temperature and a slightly different density.
  • Heating
    • The liquid expands and its density falls, so bulbs denser than the liquid sink.
  • Cooling
    • The liquid density rises, so bulbs float up.
  • Reading
    • The temperature is read from the lowest bulb that is still floating.
  • Reason
    • Liquids expand when heated, so their density decreases.

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