This higher tier covers Higher Tier: P = hρg and Upthrust within Pressure for GCSE Physics. Revise Pressure in Forces for GCSE Physics with 15 exam-style questions and 12 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 13 of 16 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Topic position
Section 13 of 16
Practice
15 questions
Recall
12 flashcards
🎓 Higher Tier: P = hρg and Upthrust
The P = hρg Formula in Detail
For a column of liquid of depth h, density ρ, and gravitational field strength g:
- Deeper depth (h) → greater pressure (more liquid above)
- Denser liquid (ρ) → greater pressure at same depth (mercury exerts far more pressure than water)
- Greater g (e.g. on Jupiter) → greater pressure (more gravitational pull on each layer)
Upthrust
When an object is submerged in a fluid, the pressure at the bottom of the object is greater than the pressure at the top (because the bottom is deeper). This pressure difference creates a net upward force called upthrust.
- Upthrust = weight of fluid displaced by the object (Archimedes' principle)
- If upthrust > weight of object: the object floats
- If upthrust < weight of object: the object sinks
- If upthrust = weight of object: the object is neutrally buoyant (e.g. a submarine hovering)
This is why ships float: they are shaped to displace enough water that the upthrust equals the ship's total weight.
Higher Tier Worked Example
Find the pressure at the bottom of the tank due to the water.
P = hρg = 5.0 × 1000 × 9.8 = 49,000 Pa
Quick Check: Why does atmospheric pressure decrease with altitude?
Because as altitude increases, there is less air above that point. The atmospheric pressure is caused by the weight of air pressing down. With less air above, the weight is smaller, so the pressure is lower.