English:Pressure in Solids, Liquids, and Gases
Pressure in Solids, Liquids, and Gases
Introduction
Pressure is all around you. Your shoes press on the ground, water pushes on the walls of a swimming pool, and the air pushes on every surface around you. In physics, pressure describes how a force is spread over an area.
This aiMOOC is designed for Grades 7–8. You will learn how pressure works in solids, liquids, and gases, how to calculate simple pressure values, how particle ideas explain gas pressure, and how pressure is used in everyday technology.
By the end, you should be able to:
- Explain pressure: Describe pressure as force acting over an area.
- Use the SI unit: State that pressure is measured in pascals.
- Compare fluids: Explain how pressure behaves in liquids and gases.
- Apply pressure ideas: Connect pressure to dams, syringes, barometers, tyres, and hydraulic machines.
Pressure in Solids
Force and Area
When a solid object presses on a surface, the pressure depends on two things: the force pushing on the surface and the area over which that force acts.
The basic relationship is:
pressure = force ÷ area
In symbols:
p = F / A
Pressure is measured in pascals, abbreviated Pa. One pascal is one newton of force spread over one square metre:
1 Pa = 1 N/m²
If the force stays the same, a smaller contact area produces a greater pressure. A larger contact area produces a smaller pressure.
For example, imagine a person standing first on flat shoes and then on shoes with very narrow heels. The person's weight is almost the same in both cases, but the narrow heels touch a much smaller area. Therefore, the pressure beneath the heels is much greater.
Why Snowshoes Work
Snowshoes spread a person's weight over a larger area. This reduces the pressure on the snow, so the person is less likely to sink deeply.
The same idea explains why tractors may use wide tyres or tracks, why a bed of nails can support a person when many nails share the force, and why a sharp knife cuts more easily than a blunt knife. In each case, the size of the contact area changes the pressure.
Example Calculation
Suppose a box pushes down with a force of 200 N on an area of 0.5 m².
p = F / A = 200 N / 0.5 m² = 400 Pa
Now imagine the same force acting on only 0.25 m²:
p = 200 N / 0.25 m² = 800 Pa
Halving the area doubles the pressure when the force stays constant.
Pressure in Liquids
Liquids are fluids, which means they can flow and take the shape of their container. A liquid at rest pushes on the bottom and sides of its container.
Pressure Increases with Depth
The deeper you go below the surface of a liquid, the greater the pressure caused by the liquid above you. A deeper point has a taller column of liquid above it, so the weight of that liquid creates more pressure.
For a liquid of nearly constant density, the pressure caused by the liquid can be written as:
p = ρgh
where ρ is the liquid's density, g is gravitational field strength, and h is depth below the surface.
For Grades 7–8, the most important idea is the pattern: greater depth means greater liquid pressure. At the same depth in the same liquid, the pressure is the same even if the container has a different shape.
This is one reason dam walls are built especially strong near the bottom. The lower parts of the wall must withstand greater water pressure.
Density Matters
At the same depth, a denser liquid produces more pressure than a less dense liquid. For example, salt water is slightly denser than fresh water, so at the same depth it produces slightly greater pressure.
Density tells you how much mass is packed into a certain volume:
density = mass ÷ volume
You do not need to memorize complex fluid calculations here. Focus on the relationship: liquid pressure increases with both depth and density.
Pressure Acts in All Directions
In a liquid at rest, pressure at a point acts in all directions. A hole in the side of a water bottle can spray water sideways because the liquid pushes on the container wall. The liquid also pushes downward on the bottom and upward on submerged surfaces.
This helps explain buoyancy: pressure is usually greater on the lower parts of a submerged object than on the upper parts, creating an upward effect.
Pascal's Principle and Hydraulics
When extra pressure is applied to a confined liquid, that pressure change is transmitted throughout the liquid. This idea is known as Pascal's principle.
Hydraulic systems use this principle. A small force on a small piston can create a much larger force on a larger piston because the same pressure acts over a larger area.
Hydraulic brakes, car lifts, excavators, and some workshop presses use pressure in liquids to transmit forces.
Pressure in Gases
Gases are also fluids, but unlike liquids they are easy to compress. Gas particles are far apart and move rapidly in random directions.
Particle Collisions Cause Gas Pressure
Gas pressure comes from particles colliding with the walls of their container. Each collision gives a tiny push. The combined effect of enormous numbers of collisions creates measurable pressure.
If more particles hit the walls each second, or if the particles hit harder, the pressure can increase.
Changing Volume
If the same amount of gas is squeezed into a smaller volume while its temperature is kept roughly constant, the particles have less space to move. They strike the container walls more often, so the pressure rises.
This pressure-volume relationship is described more formally by Boyle's law. You do not need a full gas-law calculation at this level, but you should be able to predict the direction of change: smaller volume usually means greater pressure for the same amount of gas at constant temperature.
A blocked bicycle pump gives you a practical example. When you push the handle inward, the trapped air is compressed and pushes back more strongly.
Changing Temperature
Heating a gas in a sealed rigid container makes its particles move faster on average. They collide with the walls more often and with greater effect, so the pressure rises.
Cooling the gas has the opposite effect. This is why gas containers are designed with temperature and pressure limits. You should never heat a sealed container as an experiment.
Atmospheric Pressure
The atmosphere is a layer of gas surrounding Earth. Air has mass, and gravity pulls it toward Earth. As a result, the atmosphere produces pressure.
Near sea level, standard atmospheric pressure is about 101,325 Pa, or about 101 kPa. Actual atmospheric pressure changes with weather and altitude.
A barometer measures atmospheric pressure.
At higher altitudes there is less air above you, so atmospheric pressure is generally lower. This is why air-pressure measurements can help meteorologists study weather systems and why aircraft instruments must account for changes in pressure with altitude.
Comparing Solids, Liquids, and Gases
Pressure appears in all three states, but the mechanisms are not identical.
| State | Main pressure idea | What can change the pressure? | Everyday example |
|---|---|---|---|
| Solid | A force acts over a contact area | Force and contact area | Snowshoe on snow |
| Liquid | Pressure acts through the liquid and increases with depth | Depth, density, and applied pressure | Dam wall or hydraulic brake |
| Gas | Moving particles collide with surfaces | Temperature, volume, and number of particles | Tyre, aerosol can, or bicycle pump |
A useful rule for problem solving is to ask: What is pushing, over what area, and what physical condition is changing?
Measuring Pressure
Different instruments are used for different situations.
A barometer measures atmospheric pressure. A manometer measures pressure differences in fluids, especially gases. Tyre pressure gauges measure the pressure inside a tyre compared with the surrounding atmosphere.
For many school calculations, pressure is measured in pascals. Because one pascal is quite small, larger pressures are often given in kilopascals:
1 kPa = 1000 Pa
Safe Mini-Investigations
You can explore pressure safely with simple classroom materials.
- Contact area investigation: Press the same block into soft modelling material on different faces and compare the depth of the marks.
- Bottle-hole investigation: With teacher supervision, compare how far water travels from holes at different heights in a plastic bottle.
- Syringe investigation: Use a clean needle-free syringe, block the opening with a finger, and gently compress the trapped air to feel increasing resistance.
- Weather observation: Record local barometer readings over several days and compare them with weather changes.
Never use needles, pressurized gas cylinders, sealed heated containers, or damaged pressure equipment for a school pressure experiment.
Interactive Tasks
Quiz: Test Your Knowledge
What does pressure describe in a solid contact situation? (Force spread over an area) (!Mass divided by volume) (!Distance travelled each second) (!Energy stored in a battery)
What happens to pressure if the same force acts on a smaller area? (It increases) (!It decreases) (!It becomes zero) (!It always stays the same)
What is the SI unit of pressure? (Pascal) (!Newton) (!Joule) (!Watt)
Why do snowshoes reduce sinking into snow? (They spread force over a larger area) (!They increase the person's weight) (!They remove gravity) (!They warm the snow)
How does liquid pressure change as depth increases? (It increases) (!It decreases) (!It becomes zero) (!It changes into temperature)
At the same depth, which liquid normally creates more pressure? (The denser liquid) (!The less dense liquid) (!The colder liquid in every case) (!The lighter coloured liquid)
What principle explains how pressure is transmitted through a confined liquid? (Pascal's principle) (!Newton's first law) (!Ohm's law) (!The law of reflection)
What mainly causes pressure inside a gas container? (Particle collisions with the walls) (!Particles stopping completely) (!Gravity disappearing) (!The container producing mass)
What happens to gas pressure when a sealed rigid gas is heated? (It increases) (!It always becomes zero) (!It changes into liquid pressure) (!It loses all particle motion)
Which instrument measures atmospheric pressure? (Barometer) (!Ammeter) (!Thermometer) (!Voltmeter)
Memory Game
| Pressure | Force distributed over an area |
| Pascal | SI unit used for pressure |
| Hydrostatic pressure | Pressure produced by a liquid at rest |
| Barometer | Instrument for measuring atmospheric pressure |
| Hydraulics | Use of liquid pressure to transmit force |
| Collision | Event in which a gas particle strikes a surface |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Greater contact area | Lower solid pressure for the same force |
| Greater liquid depth | Higher pressure in the same liquid |
| Denser liquid | Higher pressure at the same depth |
| Smaller gas volume | More frequent wall collisions at constant temperature |
| Barometer | Measurement of atmospheric pressure |
...
Crossword Puzzle
| Pressure | What quantity equals force divided by area? |
| Pascal | What SI unit is used for pressure? |
| Barometer | What instrument measures atmospheric pressure? |
| Hydraulics | What technology uses liquid pressure to transmit force? |
| Density | What property of a liquid helps determine its pressure at a given depth? |
| Collision | What happens when a gas particle strikes a container wall? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Pressure photo hunt: Find or photograph four everyday examples of pressure and write one sentence explaining the role of force and area in each.
- Snowshoe explanation: Draw a labelled comparison of an ordinary boot and a snowshoe on soft snow, then explain why one produces less pressure.
- Particle storyboard: Create a six-frame cartoon showing gas particles colliding with container walls and causing pressure.
- Pressure vocabulary recording: Make a one-minute audio or video explanation using the words pressure, force, area, pascal, liquid, and gas correctly.
Standard
- Contact area experiment: Design and carry out a fair test with a block and soft modelling material to investigate how contact area affects the depth of an impression.
- Liquid depth investigation: With teacher supervision, investigate water jets from holes at different heights in a plastic bottle and explain the pattern you observe.
- Weather pressure interview: Interview a science teacher, sailor, pilot, or weather enthusiast about how atmospheric pressure information is used in real situations.
- Hydraulic machine poster: Produce a poster or digital infographic explaining how a hydraulic brake, lift, or excavator uses transmitted liquid pressure.
Advanced
- Pressure calculation challenge: Create and solve three realistic pressure problems involving force and area, showing units and explaining what each answer means.
- Barometer data project: Collect several days of local atmospheric-pressure data, graph it, compare it with weather observations, and discuss any pattern you find.
- Design for low pressure: Engineer a model shoe, vehicle track, or support pad that minimizes pressure on a soft surface, then test and improve your design.
- State comparison video: Produce a three-minute educational video that compares pressure in solids, liquids, and gases and includes at least one demonstration, one diagram, and one calculation.
Learning Assessment
- Explain a design choice: A rescue vehicle must cross soft ground. Compare narrow tyres with wide tyres and justify which design should reduce sinking.
- Interpret liquid pressure: A water tank has holes near the top and near the bottom. Predict how the water jets will differ and explain your reasoning using depth.
- Transfer to hydraulics: Explain why a hydraulic lift can produce a large output force and identify the role played by pressure and piston area.
- Reason from particles: Predict what happens to the pressure of gas in a sealed rigid container when its temperature rises and explain the change using particle motion.
- Compare two states: Explain one similarity and one difference between pressure in liquids and pressure in gases.
- Evaluate evidence: A student says that a larger force always means larger pressure. Give a counterexample showing why area must also be considered.
Evidence of Learning
Strong evidence of learning includes the following:
Knowledge: You can define pressure, identify the pascal, describe how pressure depends on force and area, explain why liquid pressure increases with depth, and explain gas pressure using particle collisions.
Skills: You can calculate simple pressure values, interpret diagrams and graphs, design a fair investigation, record observations, use units correctly, and explain cause-and-effect relationships.
Products: You can create labelled diagrams, experimental reports, graphs, posters, models, interviews, or short educational videos that communicate pressure ideas accurately.
Transfer: You can apply pressure concepts to unfamiliar situations such as tyres on sand, dam design, diving, weather instruments, syringes, hydraulic machinery, and compressed gases.
OERs on the Topic
You can also explore these related open resources:
- Pressure: General background on pressure in physics.
- Hydrostatics: Pressure in fluids at rest.
- Pascal's law: Transmission of pressure through confined fluids.
- Atmospheric pressure: Pressure caused by Earth's atmosphere.
- Boyle's law: Relationship between pressure and volume for a gas under controlled conditions.
OpenStax: Fluids, Density, and Pressure provides a deeper open-access explanation for learners who want to extend beyond the Grades 7–8 level.
Linked Learning Areas
Pressure connects ideas from mechanics, states of matter, weather, engineering, measurement, and mathematical problem solving.