What mistakes do students make with Pressure in a fluid 2 (HT only)?

Writing that the upthrust on a brick lying on the bottom is zero. Saying upthrust equals the weight of the object rather than the weight of the fluid displaced. Listing upthrust as a non-contact force alongside gravity and magnetism.

What are the exam tips for Pressure in a fluid 2 (HT only) in Physics AQA GCSE?

Label the upthrust on any diagram of an object in a fluid, even a sinking one. For an object resting on the bottom, expect three vertical forces rather than two.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

Pressure in a fluid 2 (HT only): This force is called the upthrust.

Upthrust is the name for that upward resultant, and it is a contact force, so mark schemes accept it alongside tension and the normal contact force when examples are asked for.

Checked against the AQA 8463 specification (opens in a new tab) on 22 September 2026.

Point 5 of 6

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Upthrust named as the upward force from a fluid on a submerged object.
Upthrust equal to the weight of the fluid displaced.
Identifying upthrust as a contact force.
The balance of weight, upthrust and normal contact force on an object resting on the bottom.

Common mistakes and how to put them right

Writing that the upthrust on a brick lying on the bottom is zero.
Saying upthrust equals the weight of the object rather than the weight of the fluid displaced.
Listing upthrust as a non-contact force alongside gravity and magnetism.
Leaving the normal contact force out of the force balance for an object resting on the bottom.

Exam tips

Label the upthrust on any diagram of an object in a fluid, even a sinking one.
For an object resting on the bottom, expect three vertical forces rather than two.

Try the lesson

The start of the Pressure in a fluid 2 (HT only): This force is called the upthrust. lesson, made from checked teaching notes for this specification point.

Have a go first

A sealed metal block is fully submerged in water and held at rest. Which statement describes the upthrust acting on the block?

Tip: press 1 to 4 to choose

Listen

Upthrust is the name for that upward resultant, and it is a contact force, so mark schemes accept it alongside tension and the normal contact force when examples are asked for. It always acts upwards through the submerged volume and is equal in size to the weight of the fluid displaced. Whether an object rises, sinks or stays put depends on how the upthrust compares with the weight. A brick resting on the floor of a pool is held still by three forces: its weight downwards, the upthrust upwards and the normal contact force from the floor upwards, so the weight equals the upthrust plus the contact force. A helium balloon rises because the upthrust from the air exceeds its weight, and a floating object has settled where upthrust and weight are equal. A strong answer earns marks for these points. upthrust named as the upward force from a fluid on a submerged object. upthrust equal to the weight of the fluid displaced. identifying upthrust as a contact force. the balance of weight, upthrust and normal contact force on an object resting on the bottom. Exam tip: Label the upthrust on any diagram of an object in a fluid, even a sinking one.

See it

A top-down flowchart. A node labelled 'Object in a fluid' branches to 'Upthrust acts upwards' and 'Weight acts downwards'. The upthrust node leads to 'Upthrust equals weight of fluid displaced'. Both the upthrust-equals-weight-of-fluid-displaced node and the weight node lead into a decision node 'Compare upthrust with weight'. This decision branches to three outcomes: 'Upthrust exceeds weight: object rises', 'Upthrust and weight equal: object stays put', and 'Object resting on the bottom'. The last outcome branches to three forces: 'Weight downwards', 'Upthrust upwards' and 'Normal contact force upwards', and all three of these lead to a final node 'Weight equals upthrust plus contact force'. — It shows how upthrust compares with weight to decide whether an object rises, stays put or rests on the bottom, and the three-force balance for an object on the bottom.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A node labelled 'Object in a fluid' branches to 'Upthrust acts upwards' and 'Weight acts downwards'. The upthrust node leads to 'Upthrust equals weight of fluid displaced'. Both the upthrust-equals-weight-of-fluid-displaced node and the weight node lead into a decision node 'Compare upthrust with weight'. This decision branches to three outcomes: 'Upthrust exceeds weight: object rises', 'Upthrust and weight equal: object stays put', and 'Object resting on the bottom'. The last outcome branches to three forces: 'Weight downwards', 'Upthrust upwards' and 'Normal contact force upwards', and all three of these lead to a final node 'Weight equals upthrust plus contact force'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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