OCR GCSE Physics A (J249) — Matter
Every time you dive to the bottom of a swimming pool, you feel a squeezing sensation in your ears. This happens because pressure in a liquid is caused by the weight of the water column above you pushing down. As your depth increases, the weight of the liquid above you increases, which proportionally increases the pressure.
Liquid pressure acts evenly in all directions and always at right angles (perpendicularly) to any submerged surface. Crucially, the pressure at a given depth is entirely independent of the shape of the container or the surface area of the liquid; it relies only on depth, density, and gravity.
To calculate liquid pressure, you must factor in the gravitational field strength (). This is the force per unit mass exerted by a gravitational field on an object.
For OCR Gateway GCSE Physics A, you must explicitly state and use the value of as approximately near the Earth's surface. While it is numerically equivalent to the acceleration of free fall (), examiners prefer the unit when discussing pressure and fields.
The pressure exerted by a column of liquid is directly proportional to both the depth of the liquid and its density. This relationship is defined by the following equation:
Where:
This formula calculates the pressure from the liquid alone. To find the total pressure on a submerged object, you must add this value to the atmospheric pressure resting on the liquid's surface.
Differences in liquid pressure also explain upthrust. Because the bottom of a submerged object is at a greater depth than the top, the upward pressure acting on the bottom is greater than the downward pressure on the top. This pressure difference creates a resultant upward force.
You can adapt the liquid pressure formula to calculate the difference in pressure () between two vertical depths by using the change in depth ().
A research submarine descends from a depth of 40 m to 150 m in ocean water. The density of the ocean water is 1030 kg/m³. Calculate the increase in pressure experienced by the submarine. Use .
Step 1: Identify the change in depth ().
Step 2: List the required values.
Step 3: Substitute the values into the pressure difference formula.
Step 4: Calculate the final answer with units.
Students frequently forget to convert depth from centimetres (cm) to metres (m) before calculating liquid pressure, which leads to mathematically incorrect answers.
Although the liquid pressure formula is provided on the OCR equation sheet, you must actively recall that the value of g is 10 N/kg because examiners will not always provide it in the question text.
If a 3-mark or 4-mark question asks for the 'total pressure' on a submerged object, examiners expect you to calculate the liquid pressure using P = hρg and then explicitly add the atmospheric pressure to secure the final mark.
When asked to explain what causes upthrust in an exam, always use the phrase 'pressure difference between the top and bottom of the object' to secure the key conceptual marks.
Pressure
The force exerted per unit area, measured in Pascals (Pa) or Newtons per square metre (N/m²).
Depth
The vertical distance from the surface of a liquid to a specific point within it, measured in metres (m).
Gravitational field strength (g)
The force per unit mass exerted by a gravitational field, taken as approximately 10 N/kg near the Earth's surface for OCR Gateway exams.
Density
The mass per unit volume of a substance, measured in kilograms per cubic metre (kg/m³).
Atmospheric pressure
The pressure exerted by the weight of the atmosphere, approximately 100,000 Pa, which must be added to liquid pressure to find total pressure.
Upthrust
An upward force on a submerged object caused by the greater liquid pressure acting on its bottom surface compared to its top surface.
Put your knowledge into practice — try past paper questions for Physics A
Pressure
The force exerted per unit area, measured in Pascals (Pa) or Newtons per square metre (N/m²).
Depth
The vertical distance from the surface of a liquid to a specific point within it, measured in metres (m).
Gravitational field strength (g)
The force per unit mass exerted by a gravitational field, taken as approximately 10 N/kg near the Earth's surface for OCR Gateway exams.
Density
The mass per unit volume of a substance, measured in kilograms per cubic metre (kg/m³).
Atmospheric pressure
The pressure exerted by the weight of the atmosphere, approximately 100,000 Pa, which must be added to liquid pressure to find total pressure.
Upthrust
An upward force on a submerged object caused by the greater liquid pressure acting on its bottom surface compared to its top surface.
