Year 7 Science · Unit 3 · Lesson 8
Extension · Challenge Worksheet
Learning Goals
Extension · optional
Pressure sits beyond the Stage 4 Forces dot points this unit is built on, and is met properly in later years. Nothing on this sheet is assessed as part of Year 7 Forces, so it is enrichment rather than required practice. Do it if you are interested, and skip it without losing any marks if you are not.
1. Pressure depends on two things. Name them, and say what happens to the pressure when the contact area is halved but the force stays the same.
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. The pressure in water increases as you go deeper.
Correct it:
2. Snowshoes stop you sinking because they reduce the force you put on the snow.
Correct it:
3. A sharp knife cuts better than a blunt one mainly because its edge has a smaller area, not because you push harder.
Correct it:
4. Dam walls are built thickest at the base because that is where the water pushes hardest.
Correct it:
5. A heavier object always puts more pressure on the ground than a lighter one.
Correct it:
2. A red kangaroo has long, flat hind feet. An echidna has short, sharp claws that it uses to dig into hard soil for ants. Explain how both use pressure, even though one needs low pressure and the other needs high pressure.
Read the data
Scenario: bricks on foam
A class rests identical bricks on a thick block of soft foam in four different ways and measures how deep a dent each one makes. One brick weighs 20 N. The contact area depends on which face of the brick touches the foam.
| Set-up | Force on the foam (N) | Contact area (cm²) | Depth of dent (mm) |
|---|---|---|---|
| A. One brick lying flat | 20 | 200 | 3 |
| B. One brick standing on its end | 20 | 50 | 11 |
| C. Two bricks stacked, lying flat | 40 | 200 | 6 |
| D. One brick lying on its side | 20 | 100 | 6 |
Source: sample class results.
3. Which gives the higher pressure: A or B? A or C? For each, say whether you used the same-force rule or the same-area rule, and check your answer against the dents.
4. Can the same-force or same-area rule decide whether C or D gives the higher pressure? Explain, then use the dents to compare them and suggest why the result makes sense.
Someone claims
“A flat-soled boot is always safer and better than a pointed heel. Wide, flat surfaces reduce pressure, which prevents damage and injury, so designers should always spread forces over the largest possible area. Pointed or sharp surfaces are always the dangerous option.”
5. Evaluate this claim. Say which part is supported by the science of pressure, give two counterexamples where a small contact area is useful, and rewrite the claim so that it is accurate.
Question 1
Force and area (1). Halving the area with the same force increases the pressure (it doubles) (1). [2 marks]
True or false
1. True. 2. False: snowshoes do not change your weight; they increase the area, which lowers the pressure. 3. True. 4. True. 5. False: pressure depends on area too; a light stiletto heel can press harder than an elephant's foot.
Question 2
The kangaroo's flat feet spread the landing force over a large area, keeping pressure low (1). The echidna's sharp claws concentrate its digging force on a tiny area, giving a high pressure that breaks into hard soil (1). Same rule, opposite goals: the area is matched to the job (1). [3 marks]
Question 3
B: same force, smaller area; dent 11 mm vs 3 mm (1). C: same area, bigger force; dent 6 mm vs 3 mm (1). [2 marks]
Question 4
No: C has more force but also more area, so neither rule applies (1). The dents are equal, 6 mm, so the pressures are about the same (1). C has twice the force spread over twice the area, so each part of the foam carries the same share (1). [3 marks]
Question 5
Marking guide: 1 judgement (partly right, but “always” is wrong); 1 supported part with example: a large area lowers pressure and protects a surface (snowshoes, tractor tyres, camel hooves); 1 each for two counterexamples with reasons (knife edge cuts, drawing pin or tent peg pierces, echidna claws dig); 1 high pressure is the purpose in those designs; 1 rewrite: whether to spread or concentrate a force depends on the job. [6 marks]
Model answer: The claim is only partly right. Spreading a force does lower pressure, which is why snowshoes stop you sinking. But a sharp knife needs a tiny edge to cut, and a tent peg needs a point to go into the ground. Better: “Use a large area to protect a surface and a small area to cut, pierce or dig.”
Common error: treating high pressure as always harmful.