Pressure in fluids

• Pressure determines the effect of a force on a surface
• Pressure is force per unit area
• Flat shoes spread the force over a large area, reducing the pressure
• Different styles of shoes can cause different pressures due to their area
• Snow shoes have a much larger area than feet to spread the force over a larger area and reduce the pressure on the snow
• High heeled shoes transfer the force through a much smaller area, causing a much greater pressure
• Accidentally stepping on a plug or stones barefoot hurts more because the force acts on a small area and the pressure is increased
• Pressure in fluids is calculated using the equation: pressure = force normal to a surface / area of that surface
• Pressure is measured in pascals (Pa)
• Force is measured in newtons (N)
• Area is measured in metres squared (m^2)
• Example: If a fluid exerts a force of 50 N over an area of 2 m^2, the pressure on the surface is 25 Pa
• Pressure in a liquid is different at different depths
• Pressure increases as the depth increases
• The pressure in a liquid is due to the weight of the column of water above
• Since the particles in a liquid are tightly packed, the pressure acts in all directions
• The pressure acting on a dam at the bottom of a reservoir is greater than the pressure acting near the top
• Dam walls are usually wedge-shaped to accommodate the varying pressure
• The equation to calculate pressure caused by a column of liquid is: pressure = height of column × density of the liquid × gravitational field strength
• Pressure (p) is measured in pascals (Pa)
• Height of column (h) is measured in metres (m)
• Density (ρ) is measured in kilograms per metre cubed (kg/m^3)
• Gravitational field strength (g) is measured in newtons per kilogram (N/kg)
• Upthrust:

  <|>An object submerged experiences greater pressure on its bottom surface than on its top surface
• If upthrust is less than the weight of the object, the object will sink. If upthrust is larger, the object will float
• The atmosphere is the layer of air around the Earth
• Key features of the atmosphere:

  • It is thin compared to the size of the Earth
  • It becomes less dense as the altitude increases
• Atmospheric pressure is caused by air molecules colliding with a surface
• At sea level, atmospheric pressure is about 101,000 Pa (101 kPa)
• At cruising altitude of a passenger plane, atmospheric pressure is about 27,000 Pa (27 kPa)
• Atmospheric pressure decreases as the height of a surface above ground level increases because:

  • The number of air molecules decreases
  • The weight of the air decreases
  • There is less air above a surface
• If air pressure is too low at high altitudes, humans cannot take in oxygen quickly enough to meet their bodies' needs