Physics 9702 · Paper 2

Pressure

The same force can be gentle or sharp. Spread over a palm it presses lightly; behind a pin it goes through wood.
Pressure measures how concentrated a force is on an area.

The definition

p = \dfrac{F}{A}

Symbols

Pressure is the normal force per unit cross-sectional area. The word normal (at right angles) is part of the definition and carries a mark (9702/21/O/N/25 Q2(a)(i)).
The unit is the pascal: $1 \text{ Pa} = 1 \text{ N m}^{-2}$ . In base units that is kg m⁻¹ s⁻².
Pressure is a scalar. It has no direction of its own: at a point in a fluid it pushes equally on any surface (9702/22/O/N/25 Q1(a)).

Worked example

Smallest case: a box on the floor

A box pushes down with 20 N on an area of 0.50 m². Find the pressure on the floor.

Answer

p = 40 \text{ Pa}

Your turn— tap to reveal the worked answer (9702/11/O/N/25 Q15)

A man of weight 600 N stands with both feet flat on the floor. Estimate the pressure he puts on the floor. (9702/11/O/N/25 Q15)

Answer: about 1 × 10⁴ Pa. The skill being tested is the area estimate, not the division.

A block resting on its smallest face puts the largest pressure on the ground: same weight, smallest area.
Seen from the fluid side, that is also the block standing as a tallest column: pressure from a column of material is $\rho g h$ (next lesson), and the smallest face goes with the biggest height.

Worked example

Largest pressure of a lead block

A lead block (density 1.13 × 10⁴ kg m⁻³) has sides 12 cm, 15 cm and 10 cm. Find the largest pressure it can put on the ground. (9702/12/O/N/24 Q14)

Weight: $W = \rho V g = 1.13 \times 10^{4} \times (0.12)(0.15)(0.10) \times 9.81 = 200 \text{ N}$ .
Smallest face: 12 cm × 10 cm = $0.012 \text{ m}^2$ .
$p = \dfrac{200}{0.012} = 1.7 \times 10^{4} \text{ Pa}$ .

Answer

p \approx 1.7 \times 10^{4} \text{ Pa}

Pressure

The same force can be gentle or sharp. Spread over a palm it presses lightly; behind a pin it goes through wood.
Pressure measures how concentrated a force is on an area.

p = \dfrac{F}{A}

Symbols

Pressure is the normal force per unit cross-sectional area. The word normal (at right angles) is part of the definition and carries a mark (9702/21/O/N/25 Q2(a)(i)).
The unit is the pascal: $1 \text{ Pa} = 1 \text{ N m}^{-2}$ . In base units that is kg m⁻¹ s⁻².
Pressure is a scalar. It has no direction of its own: at a point in a fluid it pushes equally on any surface (9702/22/O/N/25 Q1(a)).

Worked example

Smallest case: a box on the floor

A box pushes down with 20 N on an area of 0.50 m². Find the pressure on the floor.

Answer

p = 40 \text{ Pa}

Your turn— tap to reveal the worked answer (9702/11/O/N/25 Q15)

A man of weight 600 N stands with both feet flat on the floor. Estimate the pressure he puts on the floor. (9702/11/O/N/25 Q15)

Answer: about 1 × 10⁴ Pa. The skill being tested is the area estimate, not the division.

A block resting on its smallest face puts the largest pressure on the ground: same weight, smallest area.
Seen from the fluid side, that is also the block standing as a tallest column: pressure from a column of material is $\rho g h$ (next lesson), and the smallest face goes with the biggest height.

Worked example

Largest pressure of a lead block

A lead block (density 1.13 × 10⁴ kg m⁻³) has sides 12 cm, 15 cm and 10 cm. Find the largest pressure it can put on the ground. (9702/12/O/N/24 Q14)

Weight: $W = \rho V g = 1.13 \times 10^{4} \times (0.12)(0.15)(0.10) \times 9.81 = 200 \text{ N}$ .
Smallest face: 12 cm × 10 cm = $0.012 \text{ m}^2$ .
$p = \dfrac{200}{0.012} = 1.7 \times 10^{4} \text{ Pa}$ .

Answer

p \approx 1.7 \times 10^{4} \text{ Pa}