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)
• Compression, bending or stretching an object

  Requires more than one force in different directions to deform the object
• Force applied


  Extension of an elastic object is directly proportional, provided the limit of proportionality is not exceeded
• Inelastic (plastic) deformation


  A deformation which results in the object being permanently changed, the object doesn't return to its original shape when the force is removed
• Equation relating force, spring constant and extension

  1. Force = Spring Constant x Extension

  2. Force (N), Spring Constant (N/m), Extension (m)
• Energy stored in a stretched spring

  Elastic potential energy
• Extension in spring force equation

  Can be replaced with compression
• Elastic distortion


  • Temporary change, object reverts to original shape once forces are removed
• Inelastic distortion


  • Permanent change in shape, irreversible
• Calculating work done/stored in stretching a spring

  1. Energy (J) = ½ x Spring constant (N/m) x Extension² (m²)

  2. E = ½kx²
• Linear relationship between force and extension

  When object is deforming elastically, has not reached limit of proportionality
• Nonlinear relationship between force and extension

  When limit of proportionality has been exceeded, object is undergoing plastic deformation