Pressure

The Cambridge definition has two required elements: (1) force per unit area — not "force on an area" or "divided by surface" — and (2) the unit Pa (or N/m²). Forms like "force on a surface" or "the force exerted" score zero. Correct: pressure is force per unit area; $p = F/A$; unit Pa. Write the unit as Pa or N/m² (with the ²), never "N/m".

Both $p = F/A$ and $p = h\rho g$ need SI units: depth $h$ in metres (45 cm = 0.45 m) and area $A$ in m² (50 cm² = 0.0050 m²; divide by 10 000, not 100). Using a depth in centimetres directly in $p = h\rho g$ makes the answer 100 times too large. The final mark needs the correct numerical value AND the unit Pa.

(E) $p = h\rho g$ gives only the extra pressure from the fluid column (gauge pressure). Absolute pressure = atmospheric + gauge: $p_\text{abs} = p_\text{atm} + h\rho g$, with $p_\text{atm} \approx 101\,000$ Pa. If a question provides an atmospheric value and asks for the pressure at depth, add $p_\text{atm}$. Forgetting this is the most common Extended error.

In $p = h\rho g$, $h$ is the vertical depth from the free surface down to the point of interest — not the total height of the container and not a horizontal distance. If a tank holds liquid to a height of 2 m and you want the pressure at the base, $h = 2$ m, measured from the top surface downward.

$p = F/A$ is for a solid pressing on a surface (a boot on mud); $p = h\rho g$ is for the pressure inside a fluid due to the weight of fluid above it. Both give pressure in Pa but apply to different situations. Using $p = F/A$ to find the pressure at depth in a liquid scores zero. Decide the formula from the physical context before substituting.