AQA GCSE Physics Physics Paper 2 (Higher), June 2025: Question 8

Fluid Pressure, Calculations & Upthrust

📋 What this question tests

Direction of fluid pressure: Recalling that fluid pressure produces forces acting normal (at right angles) to container surfaces.
Quantitative pressure calculation: Rearranging the equation p = F / A to solve for cross-sectional area.
Mathematical precision: Handling decimal places and expressing the final answer correctly to 3 significant figures.
Cause of upthrust: Formulating an extended, logical chain of reasoning explaining how depth and pressure differences create a resultant upward force.

Question 08.1

Direction of Liquid Force [1 Mark]

✅ Acceptable Answers

At right angles (to surfaces)
Normal (to surfaces)
Perpendicular / at 90° (to the jar walls)

💡 Key Knowledge

Particles in a liquid collide with the walls of a container in all directions. Because collisions occur randomly in enormous numbers, tangential components cancel out, leaving a net force acting at right angles (normal) to any surface the fluid touches.

❌ Common Errors

Vague directional descriptions such as "outwards", "sideways", or "in all directions" lose the mark. The specification specifically requires the geometric term normal or at right angles.

Question 08.2

Calculating Base Area from Weight and Pressure [4 Marks]

📐 Step-by-Step Calculation

Given values: Weight (Force, F ) = 1.50 N, Pressure ( p ) = 774 N/m²

1 Formula & Substitution:
p = F / A → 774 = 1.50 / A
Mark 1: Correct substitution of values into standard equation.
2 Rearrange for Area (A):
Multiply both sides by A , then divide by 774 :
A = 1.50 / 774
Mark 2: Correct rearrangement with area as the subject.
3 Calculate Raw Value:
A = 0.00193798... m²
Mark 3: Correct intermediate calculated value.
4 Round to 3 Significant Figures:
First three non-zero digits are 1, 9, 3 followed by 7 (rounds up):
A = 0.00194 m² (or 1.94 × 10⁻³ m² )
Mark 4: Correct rounding strictly to 3 significant figures.

❌ Common Calculation Pitfalls

Inversion error: Rearranging as A = 774 / 1.50 = 516 . Always remember: when the unknown is in the denominator, swap it with the left side!
Significant figure confusion: Thinking the leading zeros count. In 0.00194 , the zeros after the decimal point are place-holders, not significant figures.
Writing 0.0019 (2 s.f.) or 0.001938 (4 s.f.) forfeits the final mark.

🧠 Exam Technique: Securing 4/4

Always write down the unrounded value from your calculator display first (e.g. 0.0019379... ). If you make a rounding slip on the final answer line, the examiner can still award 3 out of 4 marks for your unrounded working.

Question 08.3

Explaining the Cause of Upthrust [4 Marks]

✅ Full-Mark Model Explanation (4-Stage Chain of Logic)

Point 1 (Depth & Pressure): Pressure in a liquid increases with depth (or vinegar has a greater density than oil).
Point 2 (Pressure Difference): The pressure exerted on the bottom surface of the garlic (by the vinegar) is greater than the pressure exerted on the top surface (by the oil).
Point 3 (Force Difference): Since F = p × A , this causes a greater upward force on the bottom of the garlic than the downward force on the top.
Point 4 (Resultant Force): The resultant (net) force of these opposing forces acts upwards, which is called upthrust.

🧠 Examiner Insights & Mark Scheme Dependencies

Mark 4 is dependent on Mark 2 or Mark 3: you cannot simply state "upthrust is the resultant force" without explaining where the force difference comes from.

Top-scoring students clearly separate their discussion into pressure first, then convert pressure into force, and conclude with the resultant direction.

❌ Misconceptions to Avoid

Only talking about density: Stating "it floats because garlic is less dense than vinegar" describes why it floats at that boundary, but does not explain why an upthrust force exists physically.
Confusing force and pressure: Using the words interchangeably (e.g. "upward pressure overcomes weight"). Pressure is force per unit area; forces cause acceleration or balance weight.