Pressure & Pressure Differences in Fluids

• What is a fluid?

  A fluid is either a liquid or a gas.
• How is pressure defined?

  Pressure is defined as the concentration of a force or the force per unit area.
• Why does a drawing pin push into a surface rather than upwards towards a finger?

  Because the sharp point creates a larger pressure due to its small area.
• What is the effect of large tires on tractors?

  Large tires spread the weight of the tractor over a large area, reducing pressure.
• How do nails work to penetrate surfaces?

  Nails have sharp pointed ends that concentrate force, creating large pressure over a small area.
• What happens when an object is immersed in a liquid?

  The liquid exerts pressure, squeezing the object.
• How do forces act on surfaces in fluids?

  Forces act at 90 degrees (right angles) to the surface.
• What units is pressure measured in?

  Pressure is measured in Pascals (Pa).
• What area should be used when calculating pressure?

  The cross-sectional area of the object where the force is at right angles to it.
• What does the pressure equation imply about force and area?

  • If a force is spread over a large area, it results in small pressure.
  • If a force is spread over a small area, it results in large pressure.

  
• Why do high heels produce higher pressure on the ground?

  Because they have a smaller area compared to flat shoes.

  
• What is the pressure exerted by the Earth's atmosphere at sea level?

  About 100 kPa.
• How does atmospheric pressure change with altitude?

  Atmospheric pressure decreases with increasing altitude.

  
• What causes atmospheric pressure?

  It is caused by air molecules colliding with a surface.
• What happens to the number of air molecules as height increases?

  The number of air molecules above a surface decreases as height increases.
• Why does atmospheric pressure decrease with altitude?

  Because there is less air above a surface at higher altitudes.

  
• What is the relationship between pressure and density in a liquid?

  Pressure in a liquid increases with the height of the column of liquid above that point.
• What causes pressure in a liquid?

  Pressure is caused by the weight of the liquid pushing against objects immersed in it.
• How does pressure change with depth in a liquid?

  Pressure increases with depth due to the increasing weight of the liquid above.
• What is the formula for calculating pressure in a liquid?

  p = h × ρ × g

  
• What do the variables in the pressure formula represent?

  p = pressure in pascals, h = height in metres, ρ = density in kg/m³, g = gravitational field strength in N/kg.

  
• How is pressure exerted in a liquid?

  Pressure is exerted evenly across the whole surface of an object in a liquid, in all directions.
• What happens to water pressure when a hole is made at the bottom of a column?

  The water will pour out with a large force due to higher pressure at the bottom.
• What is upthrust?

  Upthrust is a force that pushes upwards on an object submerged in a fluid.
• What determines the size of the upthrust on an object?

  The size of the upthrust depends on the density of the fluid and the volume of fluid displaced.
• Why do boats float?

  Boats float because their density is less than that of water.
• What causes upthrust on a submerged object?

  Upthrust is due to the difference in pressure between the top and bottom of the submerged object.
• How does pressure relate to depth in a submerged object?

  Pressure is proportional to the depth of the object.

  
• What happens to an object that is less dense than the fluid it is in?

  It will float.
• What happens to an object that is more dense than the fluid it is in?

  It will sink.
• Why does a polystyrene block float in water?

  Because polystyrene has a density of 0.05 g/cm³, which is less than the density of water.
• Why does a wooden block float in water?

  Because the density of a wooden block (0.9 g/cm³) is slightly less than the density of water.
• Why does an iron block sink in water?

  Because iron has a density (7.9 g/cm³) that is much higher than water.