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Pressure is force acting normally per unit area, p = F/A, measured in pascals when force is in newtons and area in m². The same force over a smaller area gives a higher pressure. Convert squared units carefully: 1 cm² is 0.0001 m². A pressure explanation must refer to area as well as force. The total force from a fluid on a surface depends on both pressure and the surface area.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate pressure using force and area
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What happens to pressure when the same force acts over half the area?
It doubles. Pressure is inversely proportional to area.
After watching 2: Why does a heated sealed rigid gas container develop greater pressure?
More frequent and forceful wall collisions. Temperature raises particle kinetic energy.
What you'll be able to do
Pressure is force acting normally per unit area, p = F/A, measured in pascals when force is in newtons and area in m². The same force over a smaller area gives a higher pressure. Convert squared units carefully: 1 cm² is 0.0001 m². A pressure explanation must refer to area as well as force. The total force from a fluid on a surface depends on both pressure and the surface area.
Gas particles collide with container walls and exert forces. Increasing temperature at fixed volume makes collisions more frequent and more forceful, increasing pressure. Compressing a fixed mass at constant temperature increases collision frequency at the walls. In the appropriate constant-temperature model, pV is constant. Use absolute pressure in these relationships; a pressure gauge reading may be measured relative to the atmosphere. Explain what variables are held fixed.
Liquid pressure increases with depth because of the weight of liquid above. In a uniform liquid, the pressure difference is Δp = ρgΔh. Pressure at one depth acts in all directions. A submerged object experiences a greater upward force on its bottom than downward force on its top, producing upthrust. An object floats when its weight is balanced by upthrust, not because no gravity acts on it.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Find the pressure increase 2.0 m below water’s surface using ρ = 1000 kg m⁻³ and g = 10 N kg⁻¹.
20000 Pa above surface pressure.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.