Explain why the upthrust acting on a submarine increases as it dives deeper into the ocean.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A submarine dives from the surface of the ocean to a greater depth. As it descends, the crew notice that the ballast tanks must be adjusted to maintain a steady depth.
Model answer (4 marks)
1. Pressure in a fluid increases with depth.
2. The pressure difference between the bottom and the top of the submarine therefore increases as it dives deeper.
3. This larger pressure difference produces a larger net upward force on the submarine.
4. Upthrust equals the weight of the fluid displaced; at greater depth the same volume of fluid is displaced but the surrounding fluid exerts more pressure, giving a greater upthrust.
2. The pressure difference between the bottom and the top of the submarine therefore increases as it dives deeper.
3. This larger pressure difference produces a larger net upward force on the submarine.
4. Upthrust equals the weight of the fluid displaced; at greater depth the same volume of fluid is displaced but the surrounding fluid exerts more pressure, giving a greater upthrust.
Examiner tips
- Use the term "pressure difference" to link depth and upthrust. Show the chain: depth → pressure ↑ → pressure difference ↑ → upthrust ↑. Mention the displaced volume stays constant.
- Use the phrase "weight of fluid displaced" – the key concept for upthrust.
Common mistakes
- Confusing buoyant force with upthrust. Not recognising that the displaced volume remains constant. Using vague wording like "more pressure means more lift" without explaining the pressure difference.
Mark scheme (4 marks)
- Pressure in a fluid increases with depth
- The pressure difference between the bottom and the top of the submarine increases (as the submarine dives deeper)
- This greater pressure difference produces a larger net upward force on the submarine
- Upthrust equals the weight of fluid displaced, and at greater depth the submarine displaces the same volume but the surrounding fluid exerts more pressure, giving a greater upthrust
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Pressure and pressure differences in fluids (Phys only) questions
- Explain why a sealed plastic bottle, partially filled with air, floats on the su…
- A wooden log floats on the surface of a river. Explain why the log floats rather…
- A hot air balloon rises through the atmosphere. Explain why the upthrust acting …
- Explain why a diver experiences greater pressure when swimming at the bottom of …