GCSE Physics revision

Pressure in a fluid 2 (HT only)

Pressure in a fluid

AQA 4.5.5.1.2

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AQA student objectives

Learning pathway

ALL 🎯

Explain why, in a liquid, pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid.

MOST 🎯🎯

Apply the scientific explanation of pressure in a fluid 2 to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving pressure in a fluid 2 and explain the scientific reasoning.

Revision summary

Read on screen, then print for Cornell-style active revision.

Pressure in a liquid is caused by the constant collisions of surrounding water molecules against a submerged object.
A component of the pressure is also due to the weight of the liquid above the object, acting as a downward force.
As an object moves deeper, the pressure increases because there is more liquid above it.

1. What Causes Pressure in a Liquid?AQA

Pressure in a liquid is caused by the constant collisions of surrounding water molecules against a submerged object.
A component of the pressure is also due to the weight of the liquid above the object, acting as a downward force.
As an object moves deeper, the pressure increases because there is more liquid above it.

The depth of the object affects pressure — the greater the depth, the greater the pressure.
The density of the liquid affects pressure — a denser liquid has greater mass per unit volume, so a greater weight.
The gravitational field strength affects pressure — a stronger gravitational field increases the weight of the liquid above the object.

2. Factors Affecting Liquid PressureAQA

The depth of the object affects pressure — the greater the depth, the greater the pressure.
The density of the liquid affects pressure — a denser liquid has greater mass per unit volume, so a greater weight.
The gravitational field strength affects pressure — a stronger gravitational field increases the weight of the liquid above the object.

Pressure in a liquid is calculated using the equation: P = hρg , where P is pressure in pascals (Pa), h is the depth in metres, ρ is the density in kg/m³, and g is the gravitational field strength in N/kg.
On Earth, the gravitational field strength is taken as g = 9.8 N/kg.
The equation shows that pressure increases proportionally with depth, density, and gravitational field strength.

3. The Pressure in a Liquid EquationAQA

Pressure in a liquid is calculated using the equation: P = hρg , where P is pressure in pascals (Pa), h is the depth in metres, ρ is the density in kg/m³, and g is the gravitational field strength in N/kg.
On Earth, the gravitational field strength is taken as g = 9.8 N/kg.
The equation shows that pressure increases proportionally with depth, density, and gravitational field strength.

At a depth of 20 m, the pressure is 20 × 1000 × 9.8 = 196,000 Pa.
At a depth of 90 m, the pressure is 90 × 1000 × 9.8 = 882,000 Pa.
The change in pressure is 882,000 − 196,000 = 686,000 Pa, which can also be found using the depth difference: 70 × 1000 × 9.8 = 686,000 Pa.

4. Worked Example: Calculating Change in PressureAQA

At a depth of 20 m, the pressure is 20 × 1000 × 9.8 = 196,000 Pa.
At a depth of 90 m, the pressure is 90 × 1000 × 9.8 = 882,000 Pa.
The change in pressure is 882,000 − 196,000 = 686,000 Pa, which can also be found using the depth difference: 70 × 1000 × 9.8 = 686,000 Pa.

Upthrust is the resultant upward force exerted on a submerged object by the surrounding liquid.
Because the bottom of a submerged object is deeper than the top, the upward force on the bottom is greater than the downward force on the top, producing a net upward force.
All submerged objects experience upthrust, regardless of whether they float or sink.

5. Understanding UpthrustAQA

Upthrust is the resultant upward force exerted on a submerged object by the surrounding liquid.
Because the bottom of a submerged object is deeper than the top, the upward force on the bottom is greater than the downward force on the top, producing a net upward force.
All submerged objects experience upthrust, regardless of whether they float or sink.

Whether an object floats or sinks depends on the relative sizes of the upthrust (upward) and the object's weight (downward).
If the weight of the object is greater than the upthrust, the object sinks.
If the upthrust is greater than the weight of the object, the object floats.

A simple rule is that if an object is denser than the liquid it is placed in, it will sink.
If an object is less dense than the liquid, it will float — for example, an apple floats in water because it is less dense than water.
Comparing the density of the object to the density of the liquid removes the need to calculate upthrust and weight separately.

7. Using Density to Predict Floating or SinkingAQA

A simple rule is that if an object is denser than the liquid it is placed in, it will sink.
If an object is less dense than the liquid, it will float — for example, an apple floats in water because it is less dense than water.
Comparing the density of the object to the density of the liquid removes the need to calculate upthrust and weight separately.