Pressure & Pressure Differences in Fluids

• What is a fluid?

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

  Pressure is the concentration of a force or the force per unit area.

  
• What happens when a drawing pin is pushed downwards?

  It is pushed into the surface rather than up towards the finger.

  
• Why does a sharp point create larger pressure when pushing a drawing pin?

  Because the sharp point has a smaller area, creating a larger pressure.
• What is the effect of large tires on tractors?

  • Spreads the weight (force) over a large area
  • Reduces pressure
  • Prevents the heavy tractor from sinking into the mud
• How do nails work in terms of pressure?

  • Nails have sharp pointed ends with a very small area
  • Concentrates the force
  • Creates a large pressure over a small area
  • Allows the nail to be hammered into a wall
• 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?

  These forces act at 90 degrees (at right angles) to the surface.
• What does the pressure of a fluid on an object create?

  A force normal (at right angles) to the surface.
• What are the units of pressure?

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

  The cross-sectional area of the object.
• 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 it 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 created by the pump in the lifting machine example?

  The pump creates a pressure of $5.28 \times 10^5 \text{ Pa}$.
• How do you calculate the force that the liquid applies to the piston?

  Using the equation $F =$$p \times A$.
• What is the calculated force applied to the piston in the lifting machine example?

  The force is $14,400 \text{ N} =$$14.4 \text{ kN}$.
• What is the atmospheric pressure at sea level?

  About 100 kPa.
• How does the atmosphere change with altitude?

  The atmosphere becomes less dense with increasing altitude.

  
• What happens to atmospheric pressure as altitude increases?

  The atmospheric pressure decreases with an increase in height.

  
• What causes atmospheric pressure?

  Air molecules colliding with a surface create atmospheric pressure.

  
• How does the number of air molecules above a surface change with height?

  The number of air molecules decreases as the height increases.

  
• What is the relationship between atmospheric pressure and air density?

  Atmospheric pressure decreases as the density of the molecules decreases.

  
• What factors affect pressure in a liquid?

  • Pressure increases with the height of the column of liquid.
  • More liquid above a point increases pressure.
  • Pressure is caused by the weight of the liquid pushing against objects.
  • Pressure increases with depth in a liquid.
• Why does pressure increase with depth in the ocean?

  Because the amount of liquid (and hence the weight) increases with depth.
• What is the formula to calculate pressure in a liquid?

  $p =$$h \times \rho \times g$
• What do the variables in the pressure equation represent?

  $p =$$\text{pressure in pascals (Pa)}, h =$$\text{height in metres (m)}, \rho =$$\text{density in kg/m}^3, g =$$\text{gravitational field strength in N/kg}$
• How is pressure exerted in a liquid?

  The force from the pressure is exerted evenly across the whole surface of an object in a liquid.
• What happens to pressure at different depths in a liquid?

  The pressure changes with the depth in a liquid.
• What is upthrust and how does it work?

  • Upthrust is a force that pushes upwards on an object submerged in a fluid.
  • It is opposite to the object's weight.
  • The size of upthrust depends on the fluid's density and the volume of fluid displaced.
  • Upthrust is due to the difference in pressure between the top and bottom of the submerged object.
• Why do boats float?

  Because they are less dense than water, resulting in upthrust greater than their weight.
• What determines whether an object sinks or floats?

  If the upthrust is equal to or greater than the object's weight, it will float; otherwise, it will sink.
• How does the density of an object affect its ability to float?

  If the object's density is less than the fluid's density, it will float; if more, 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 (1.0 g/cm³).
• Why does a wooden block float partially submerged in water?

  Because the density of the 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.