Topic 15

• Fluid


  A liquid or a gas
• Pressure in a fluid

  The forces due to pressure act at right angles (normal to) the surface
• Calculating pressure


  1. Pressure (Pa) = Force (N) / Area (m²)

  2. P = F / A
• Atmospheric pressure and height above Earth's surface

  The number of air molecules decreases the higher you go, meaning the weight of air above a point decreases, so the pressure also decreases
• The pressure in a fluid is determined by the fluid and the atmospheric pressure
• Direction of pressure


  Pressure acts in all directions
• How pressure in fluids increases with depth (Higher)

  As the depth increases, the mass of the liquid above the point also increases, so the force produced by that mass increases. Since the force has increased whilst the area has remained constant, the pressure will increase.
• Why pressure in fluids increases with density (Higher)

  As density increases, there are more particles in a given volume of the liquid, hence the weight of the liquid is increased. This means that the force of liquid above a certain point is larger. Since the force has increased, the pressure must also increase.
• Equation for pressure in liquids at different depths (Higher)

  1. Pressure (Pa) = Column height (m) x density (kg/m³) x gravitational field strength (N/kg)

  2. P = hρg
• Upthrust on objects in a fluid (Higher)

  When an object is submerged in fluid, it experiences a higher pressure below it than on top of it, leading to an upwards force called upthrust
• Magnitude of upthrust (Higher)

  Upthrust is equal to the weight of fluid displaced by the object
• Determining if an object will sink or float (Higher)

  If the upthrust is greater than the weight of the object, it will float. If the weight is the greater force, it will sink.
• An object with a density more than that of water would never float (Higher)