Last reviewed: 2 November 2026
Updated November 2026. These Grade 8 notes on pressure cover sub-strand 3.2 of Grade 8 Integrated Science: what pressure is, pressure in solids and how it depends on weight and area of contact, pressure in liquids and how it changes with depth, and everyday uses from syringes and drinking straws to car brakes, bicycle pumps and siphons. Worked examples and short practice questions with answers are included.
The scope follows the Pressure sub-strand of the KICD Grade 8 Integrated Science curriculum design (Strand 3.0, Force and Energy, about 16 lessons). The design asks for a qualitative treatment of pressure and liquid depth only, so there are no liquid-pressure formulas to memorise at this level.
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What Is Pressure? Grade 8 Notes on Pressure Basics
Pressure is the force acting on a unit area of a surface. The same force can produce a small or a large pressure depending on how much area it is spread over.
Pressure = Force ÷ Area
Force is measured in newtons (N) and area in square metres (m²), so pressure is in newtons per square metre (N/m²), also called pascals (Pa). For a solid resting on a surface, the force is its weight.
Pressure in Solids
Two factors decide the pressure a solid exerts:
- Weight (force): a heavier object exerts more pressure on the same area.
- Area of contact: the smaller the area, the greater the pressure for the same weight.
Classroom activity: press a brick into sand or soft soil, first on its largest face, then on its smallest face. The brick sinks deeper on the small face, because the same weight acts on a smaller area.
Worked example 1: a box weighs 800 N. Its faces measure 2 m², 1 m² and 0.5 m². Pressure on the largest face = 800 ÷ 2 = 400 N/m². On the smallest face = 800 ÷ 0.5 = 1,600 N/m². The pressure is greatest when the box stands on its smallest face.
Worked example 2: a learner weighing 500 N stands on both feet with a total area of 0.025 m². Pressure = 500 ÷ 0.025 = 20,000 N/m². Standing on one foot halves the area, so the pressure doubles to 40,000 N/m².
Pressure in Liquids
A liquid presses on the bottom and sides of its container and on anything placed in it. The design asks learners to observe three things:
- Pressure increases with depth. Make three holes at different heights on a plastic bottle and fill it with water. The jet from the lowest hole goes furthest, showing the greatest pressure is at the bottom.
- Pressure acts in all directions at the same depth. Holes at the same height on different sides of the bottle give jets of equal length.
- Pressure in an enclosed liquid is passed on equally. Squeeze a closed plastic bag of water with pin holes and water squirts out of every hole.
This explains why dam walls, such as those of the hydropower dams on the Tana River, are built thicker at the bottom, and why water tanks are raised high on stands so that water flows to the taps with force.
Applications of Pressure in Solids and Liquids
| Application | How pressure is used |
|---|---|
| Cutting tools (knives, pangas, axes) | A sharp edge has a tiny area, so a small push gives a large pressure that cuts easily. |
| Needles, nails and drawing pins | Pointed tips concentrate force on a very small area. |
| High-heeled shoes | The narrow heel has a small area, so it exerts a high pressure and can damage soft floors. |
| Wide tyres, broad bag straps, snow shoes | A large area spreads the weight and reduces pressure, so tractors do not sink in mud and straps do not hurt. |
| Syringe | Pushing the plunger applies pressure to the liquid, which is passed on and forces it out through the needle. |
| Car brakes | Pressing the pedal creates pressure in the brake fluid, which is passed on to all the wheels to push the brake pads. |
| Bicycle pump | Pushing the handle increases the pressure of the air inside, forcing it into the tyre. |
| Drinking straw | Sucking lowers the pressure inside the straw, so the higher air pressure on the drink pushes it up. |
| Siphon | A tube full of liquid moves it from a higher container to a lower one, for example emptying a fish tank or drawing fuel. |
Practice Questions With Answers
- State the SI unit of pressure. Answer: the pascal (Pa), which is one newton per square metre.
- A block of weight 120 N rests on an area of 0.4 m². Find the pressure. Answer: 120 ÷ 0.4 = 300 N/m².
- Why does a sharp knife cut better than a blunt one? Answer: its edge has a smaller area, so the same force gives a greater pressure.
- Water spurts out furthest from the lowest hole in a bottle. What does this show? Answer: liquid pressure increases with depth.
- Why are tractor tyres wide? Answer: to increase the area of contact and reduce pressure, so the tractor does not sink into soft ground.
Common Mistakes
- Multiplying force by area instead of dividing.
- Using an area in cm² without converting to m² (1 m² = 10,000 cm²).
- Saying a larger area gives a larger pressure. It gives a smaller pressure.
- Saying liquid pressure acts only downwards. It acts in all directions.
Where Pressure Fits in Grade 8 and KJSEA
Pressure is the last sub-strand of Grade 8 Integrated Science, after transformation of energy. KJSEA at the end of Grade 9 examines all of junior school science: the KNEC KJSEA regulations set two Integrated Science papers, 905/1 and 905/2, so a pressure calculation can appear as a structured question. For the whole year, read the Grade 8 Integrated Science revision guide, then look ahead to the Grade 9 notes on waves.
Practise free on the Grade 8 CBC Challenge, sit the timed Grade 8 Integrated Science practice exam, or browse the Grade 8 notes hub.
Frequently Asked Questions
What is the formula for pressure in Grade 8?
Pressure = force ÷ area. With force in newtons and area in square metres, the answer is in N/m², which is the same as pascals (Pa).
What factors affect pressure in solids?
The weight (force) of the object and the area of contact. More weight gives more pressure; a larger area gives less pressure.
Do Grade 8 learners calculate pressure in liquids?
No. The Grade 8 design treats the link between liquid pressure and depth qualitatively: learners show that pressure increases with depth and acts in all directions, without a formula.
What are the applications of pressure in daily life?
Sharp cutting tools, needles, wide tyres and bag straps, syringes, car brakes, bicycle pumps, drinking straws and siphons all depend on pressure.
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