Pressure: Force and Area

Overview

Students investigate how the same force can produce different pressures when it acts over different contact areas. They build on prior learning about forces and fair testing, then apply the relationship between force, area and pressure to everyday tools such as an axe and a hammer.

Learning intentions

Students will:

• describe pressure as force acting perpendicular to a surface divided by contact area
• explain how changing force or contact area changes pressure
• plan and carry out a fair investigation using observations and measurements
• apply evidence about pressure to explain why tools are designed with particular surfaces

Success criteria

• I can state the relationship between pressure, force and area.
• I can predict whether a larger or smaller area will create greater pressure when the force stays the same.
• I can identify variables and use evidence from an investigation to support a conclusion.
• I can explain why an axe blade concentrates force more effectively than a hammer face for cutting.

Curriculum links

• Physical Science: pressure is perpendicular force divided by the area over which the force acts.
• Physical Science: pressure depends on both the total applied force and the total area.
• Physical Science: smaller areas produce greater pressure with the same force; larger areas spread force and reduce pressure.
• Science capabilities in the refreshed curriculum: investigating, interpreting evidence, communicating explanations and considering how scientific ideas inform designed tools.

Lesson structure (90 minutes)

1. 0–8 min · Hook and prediction. Display a close-up comparison of an axe blade and hammer face using the hook comparison slide. Ask: “If the same person applies the same force, which tool will make a deeper mark, and why?” Students make an individual prediction, then discuss with a partner. Collect several explanations without confirming the answer.

2. 8–20 min · Build the concept. Use the pressure teaching slides to establish that pressure is force divided by area and that the force must act perpendicular to the surface. Model the qualitative relationships: same force plus smaller area means greater pressure; same force plus larger area means lower pressure. Students complete the key diagram and sentence starters on the pressure investigation worksheet. Clarify that pressure is not the same as force.

3. 20–30 min · Fair-test planning. Demonstrate pressing a block of wood or foam with its broad face and then its narrow face. Discuss what must be kept the same: object, approximate applied force, surface and pressing time. Students work in groups of three as force measurer, observer/recorder and equipment manager. They predict which orientation will create the deeper indentation and identify the independent, dependent and controlled variables on the worksheet.

4. 30–53 min · Investigation: concentrating force. Open the investigation instructions and provide each group with a soft modelling material, a rectangular block or eraser, a ruler and a tray. Students press the same object onto the material using two different contact areas, taking care to use a similar force each time. They measure or compare the indentation, repeat each condition at least twice, record results and calculate contact area where dimensions allow. Circulate to question: “What evidence suggests the force was similar?” and “What changed?”

5. 53–65 min · Analyse and explain. Groups use the data discussion slides to compare results. Students complete the results table and write a conclusion using: “When the force stayed approximately the same, changing the area from ___ to ___ caused pressure to ___ because …”. Groups identify one limitation, such as inconsistent force or difficulty measuring the indentation, and suggest an improvement.

6. 65–80 min · Apply to tools and contexts. Show images of an axe blade, hammer face, drawing pin, snowshoe and knife in the application slides. Pairs explain whether each design concentrates or spreads force and why. They answer the worksheet question: “Why is an axe blade effective for splitting wood, while a hammer face is better for striking without cutting?” Invite students to connect the explanation to contact area rather than simply saying one tool is “stronger”.

7. 80–90 min · Plenary and assessment. Return to the opening prediction using the plenary slide. Students complete the final worksheet response independently: “A force of the same size acts on two surfaces. Surface A has half the contact area of Surface B. Which experiences greater pressure? Explain.” Take three responses, address misconceptions, and collect worksheets.

Resources

• the complete pressure slide deck
• the pressure investigation worksheet
• Soft modelling material, clay, plasticine or foam
• Identical blocks, erasers or rectangular objects with different faces
• Rulers and trays
• Balance or force meter, if available
• Safety glasses and table wipes
• Board and markers

Assessment

• Listen to predictions and questioning during the hook and planning discussion; check whether students distinguish force from pressure.
• During the investigation, assess variable selection, repeated observations and use of evidence in group conclusions.
• Use the final worksheet response to assess whether students can explain the inverse relationship between contact area and pressure. Look for the idea that, with the same force, half the area produces greater pressure.

Differentiation

• Support students with the worksheet sentence starters, a labelled force–area diagram and the word bank: force, area, contact, perpendicular, pressure, concentrate and spread.
• Pair students strategically and assign practical roles. Allow students who find writing difficult to record results in a table or give an oral explanation before writing.
• For EAL learners, preview the tool images and demonstrate “same force”, “smaller area” and “greater pressure” physically; accept labelled diagrams alongside written explanations.
• Extend confident students by asking them to use (P = F ÷ A) with supplied values, compare two tool designs, or propose a test that measures force more reliably.