10 min readConcept sheetSection A MCQ (1 mark) and fill in the blanks (2 marks, two parts); Section B very short answer (2 marks) or short answer numerical (3 marks)

Thrust and pressure

Thrust is a force acting perpendicular to a surface. Pressure is thrust per unit area, so a smaller area gives more pressure.

ThrustA force acting perpendicular (at right angles) to a surface; SI unit newton (N)

PressureP = F/A (F = thrust in N, A = area in m²); SI unit N m⁻², called the pascal (Pa)

One pascal1 Pa = a thrust of 1 N acting on an area of 1 m²

RuleSame force on a smaller area gives more pressure; on a larger area, less pressure

Thrust and pressure

A bucket of water on the floor presses down with its weight. This force acts at right angles to the floor. A force acting perpendicular to a surface is called thrust. The effect of a thrust depends on the area it acts on. Pressure is the thrust on unit area: P = F/A. Its SI unit is N m⁻², named the pascal (Pa) after Blaise Pascal.

Increasing or reducing pressure

Small area, large pressureKnives, axes and blades have sharp edges. Nails, needles and drawing pins have pointed tips. The tiny area gives a large pressure, so they cut or go in easily.

Large area, small pressureArmy tanks run on wide continuous chain belts. Heavy trucks have wide tyres. Buildings and dams stand on wide foundations. School bags have broad straps. The large area spreads the force and lowers the pressure.

A camel walks easily on sand because its broad, padded feet spread its weight over a large area. A porter puts a round cloth pad on his head for the same reason.

Worked example: pressure of a brick

Problem

A brick weighs 30 N. Its faces measure 0.2 m × 0.1 m, 0.2 m × 0.05 m and 0.1 m × 0.05 m. Find the pressure on the ground when it rests on its largest face, and on its smallest face.

Working

Thrust F = weight = 30 N in both cases.
Largest face: A = 0.2 m × 0.1 m = 0.02 m². P = F/A = 30 N ÷ 0.02 m² = 1500 Pa.
Smallest face: A = 0.1 m × 0.05 m = 0.005 m². P = 30 N ÷ 0.005 m² = 6000 Pa.

Answer

1500 Pa on the largest face and 6000 Pa on the smallest. The smallest face has a quarter of the area, so it gives four times the pressure

Activity (practical): pressure of an iron cuboid on sand

You need a tray of fine dry sand or wheat flour, an iron cuboid of about 1 kg, a spring balance, thread and a half metre scale.

1
Level the fine sand (or wheat flour) in the tray without pressing it down.
2
Hang the cuboid from the spring balance by a thread and note its weight in newtons. This is the thrust.
3
Measure the length l, breadth b and height h of the cuboid with the scale.
4
Place the cuboid gently on the sand on its largest face. Note how deep it sinks.
5
Smooth the sand and repeat on each of the other two faces.
6
Find each face's area (l × b, l × h, b × h) in m², and each pressure as weight ÷ area. Record them in a table.
7★
Observe: the cuboid sinks deepest on its smallest face, where the pressure is greatest, and least on its largest face.

★ key step

Worked example: the cuboid in the practical

Problem

An iron cuboid of mass 1.2 kg measures 10 cm × 5 cm × 3 cm. Find the pressure it exerts on the sand when it rests on its 10 cm × 5 cm face. (g = 9.8 m s⁻²)

Working

Thrust = weight = mg = 1.2 kg × 9.8 m s⁻² = 11.76 N.
Area = 10 cm × 5 cm = 50 cm² = 50 × 10⁻⁴ m² = 0.005 m².
Pressure = F/A = 11.76 N ÷ 0.005 m² = 2352 Pa.

Answer

About 2350 Pa (2.35 × 10³ Pa), rounded to three significant figures

In real life

Vegetable sellers keep their knives sharp so that a small push cuts easily. Tractors use wide rear tyres so that they do not sink into soft, wet fields.

Exam angle

2 marks: “Define thrust and pressure. Give the SI unit of each.” Thrust is the force acting perpendicular to a surface; unit newton (N). Pressure is the thrust on unit area; unit N m⁻², called the pascal (Pa).

2 marks: “Why does an army tank move on wide chain belts and not on ordinary wheels?” The wide belts give a large area of contact. The tank's huge weight is spread over this area, so the pressure on the ground is small and the tank does not sink into soft ground.

3 marks: “A box of weight 600 N rests on the floor on a base of 0.5 m × 0.4 m. Find the pressure on the floor. What is the pressure if it is turned onto a base of 0.4 m × 0.25 m?” Area 1 = 0.5 × 0.4 = 0.2 m², so P = 600 ÷ 0.2 = 3000 Pa. Area 2 = 0.4 × 0.25 = 0.1 m², so P = 600 ÷ 0.1 = 6000 Pa. Halving the area doubles the pressure.

Remember it

Pointed to pierce, wide to spread: P = F/A.