Pressure: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Pressure revision notes

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Section A

1. State the equation for pressure and give its SI unit. [2]

2. State the equation for the pressure due to a column of liquid, defining each term. [3]

Section B

3. A block of mass 12 kg has a base measuring 0.30 m × 0.20 m. Take g = 10 N kg⁻¹.

(a) Calculate the pressure it exerts on the ground. [3] (b) The block is stood on a smaller face measuring 0.20 m × 0.15 m. Calculate the new pressure and explain the change. [3]

4. (Cross-topic — the particle explanation of gas pressure is a thermal physics/kinetic theory outcome, not part of this pressure subtopic.) Explain, in terms of particles, why a gas exerts pressure, and why the pressure rises when the volume is reduced at constant temperature. [4]

5. A diver descends to 25 m in water of density 1030 kg m⁻³. Take g = 9.81 N kg⁻².

(a) Calculate the pressure due to the water alone. [3] (b) Calculate the total pressure, taking atmospheric pressure as 101 000 Pa. [1] (c) Explain why the pressure does not depend on the surface area of the diver. [2]

6. (Background, not examinable — hydraulic systems and transmission of pressure are beyond the O Level Physics 5054 pressure specification.) In a hydraulic braking system, a force of 40 N is applied to a piston of area 0.0004 m².

(a) Calculate the pressure in the fluid. [2] (b) The output piston has an area of 0.0060 m². Calculate the output force. [3] (c) Explain why the system uses a liquid and not a gas. [2]

7. Explain why a sharp knife cuts more easily than a blunt one under the same applied force, and why snowshoes stop a person sinking into snow. [3]

8. Describe an experiment showing that pressure in a liquid acts at right angles to a surface, regardless of the surface’s orientation. [3]

9. Explain how a liquid barometer measures atmospheric pressure, and why mercury rather than water is used in practice. [4]

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Answers

1. pressure = force ÷ area [1]; the unit is the pascal (Pa), equal to 1 N m⁻² [1].

2. p = ρgh [1], where ρ is the density of the liquid (kg m⁻³), g is the gravitational field strength (N kg⁻¹) and h is the depth below the surface (m) [1] [1].

3. (a) Weight = 12 × 10 = 120 N [1]; area = 0.060 m² [1]; p = 120 ÷ 0.060 = 2000 Pa [1]. (b) Area = 0.030 m² [1]; p = 120 ÷ 0.030 = 4000 Pa [1]; the force is unchanged but the area is halved, so the pressure doubles [1].

4. Gas particles are in constant random motion and collide with the container walls [1]; each collision exerts a force, and force per unit area is pressure [1]. Reducing the volume means the particles have less distance to travel between collisions [1], so collisions with the walls are more frequent, and the pressure rises [1].

5. (a) p = ρgh = 1030 × 9.81 × 25 [1] [1] = 253 000 Pa (2.53 × 10⁵ Pa) [1]. (b) Total = 253 000 + 101 000 = 354 000 Pa [1]. (c) Pressure at a point in a liquid depends only on depth, density and g [1]; a larger area experiences a proportionally larger force, so the force per unit area is unchanged [1].

6. (a) p = 40 ÷ 0.0004 [1] = 100 000 Pa [1]. (b) Pressure is transmitted equally throughout the fluid [1]; F = p × A = 100 000 × 0.0060 [1] = 600 N [1]. (c) Liquids are virtually incompressible, so the applied pressure is transmitted immediately and in full [1]; a gas would compress first, so movement of the input piston would not produce an immediate output force [1].

7. A sharp knife has a much smaller contact area than a blunt one, so for the same force the pressure is much higher, making it cut more easily [1]. Snowshoes have a much larger contact area than boots, so the same body weight is spread over a larger area, producing a much lower pressure and preventing sinking [1]. Both illustrate that pressure depends on force and area, not force alone [1].

8. Connect a thistle funnel to a manometer and submerge the funnel’s open end (covered by a thin membrane) at a fixed depth in a liquid [1]. Rotate the funnel to face different directions while keeping it at the same depth [1]. The manometer reading stays the same regardless of orientation, showing the liquid pushes perpendicular to the membrane whichever way it faces [1].

9. A liquid barometer uses a tube of liquid, closed at the top (with a vacuum above) and open at the bottom to a reservoir exposed to the atmosphere [1]; atmospheric pressure pushes down on the reservoir, supporting a column of liquid whose height is a direct measure of atmospheric pressure [1]. Mercury is used because it is roughly 13.6 times denser than water [1], so it produces the same supporting pressure with a much shorter, more manageable column than water would need [1].

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Where marks are usually lost

• Using mass instead of weight when calculating pressure on the ground.
• Forgetting to add atmospheric pressure when total pressure is asked for.
• Saying pressure in a liquid depends on the volume of liquid above.
• Explaining hydraulic systems without stating that pressure is transmitted equally.
• Explaining the knife/snowshoe examples by force alone, without mentioning the change in contact area.
• Describing the barometer experiment without keeping the depth fixed while the orientation changes.