Pressure — Qualitative
Engineers designing the 2023 Snowy 2.0 tunnel had to calculate exactly how 260,000 tonnes of rock above exerts pressure on every square metre of concrete lining — get the maths wrong and the tunnel collapses.
Printable Worksheets
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Q1 · Why is it easier to cut with a sharp knife than a blunt one — the force you use is about the same?
Q2 · Would you rather be stepped on by an elephant (3,000 kg, flat wide feet) or someone in stiletto heels (60 kg, tiny tip)? Why?
● Know
- Pressure = the effect of a force spread over an area
- Smaller area → more pressure (same force)
- Larger area → less pressure (same force)
● Understand
- Why a sharp knife cuts better than a blunt one
- Why snowshoes stop you sinking into snow
- Why pressure in fluids increases with depth
● Can do
- Predict which situation produces more pressure given the same force
- Rank scenarios by pressure (qualitative)
- Explain engineering designs using pressure concepts
A drawing pin has a flat head and a sharp point. When you push on the flat head, your thumb is fine. The sharp point goes into the wall. Same force — completely different pressure.
Pressure is how much force is concentrated on a given area:
- Smaller area → pressure increases (same force squashed into a tiny spot)
- Larger area → pressure decreases (same force spread thinly)
Think of it like butter on toast. Press the flat of a knife down — the butter barely moves. Turn the knife on its edge and press — it cuts straight through. The force from your hand is the same. The area is completely different.
Year 7 key point: You do NOT need the formula P = F/A right now (that's Stage 5). What matters today is the concept: same force, smaller area → bigger pressure.
| Situation | Area | Pressure | Effect |
|---|---|---|---|
| Snowshoes | Large | Low | Don't sink into soft snow |
| High heels (stilettos) | Tiny | Very high | Sink into soft ground, damage floors |
| Elephant flat foot | Large | Moderate | Spreads weight well — less damage per cm² |
| Sharp knife edge | Tiny | Very high | Cuts through food easily |
| Blunt knife edge | Wider | Lower | Doesn't cut — force is spread out |
The stiletto heel result surprises most people: a 60 kg person in stilettos can exert more pressure on a floor than a 3,000 kg elephant. The elephant weighs 50× more — but its feet spread the force over a huge area. The stiletto tip has a contact area of less than 1 cm². Same rule: same (or even less) force, tiny area → enormous pressure.
Pressure in fluids: In water, pressure increases as you go deeper. The deeper you dive, the more weight of water is pressing down on you from above. That's why submarine hulls must be incredibly strong, and why dams have thicker walls at the base.
Engineers and nature both use pressure principles constantly:
- Dams: Water pressure increases with depth — the deeper you go, the more force the water pushes outward. That's why dams are built much thicker at the base than the top. The base must withstand the greatest pressure.
- Camel hooves: Wide and soft. Desert sand is soft — a narrow hoof would sink in. Wide hooves distribute the camel's weight over a large area, keeping pressure low enough to walk on sand.
- Tractor tyres: Wide tyres on farm tractors spread the vehicle's weight over a large area. This prevents the tyres from compacting (squashing down) soft agricultural soil, which would damage the crops.
- Red kangaroos: Large, flat hind feet help distribute impact pressure when they land from big jumps. Natural shock absorbers working on pressure principles.
- Hydraulics (preview): Car brakes use pressure in fluids. A small force on the brake pedal creates pressure in the brake fluid, which is transmitted to all four brakes with much greater force. Pressure in fluids pushes equally in all directions.
Wrong: "A heavier object always creates more pressure." Not true — pressure depends on force AND area. A lighter object on a tiny area can create more pressure than a heavier object on a large area (stilettos vs elephant).
Right: Pressure depends on both force and area. Lighter person, tiny stiletto tip → more pressure than heavier elephant with wide flat feet.
Wrong: "Snowshoes reduce the force you apply." Snowshoes don't change your weight at all. They increase the area over which your weight is spread, reducing pressure on the snow.
Right: Snowshoes don't change force — they increase area. Lower pressure = snow doesn't collapse under you.
Wrong: "Dams are thick at the top because that's where the structure is tallest." Dams are thicker at the base because water pressure is greatest at the bottom — the weight of all the water above pushes hardest there.
Right: Dam walls are thickest at the base. Pressure in water increases with depth — greatest pressure is at the bottom.
An elephant (mass 3000 kg) stands on 4 large flat feet (total area ~2000 cm²). A person in stiletto heels (mass 60 kg) stands on one heel tip (area ~1 cm²). Predict: who creates more pressure on the floor — the elephant or the stiletto heel? Justify your prediction.
How close was your prediction?
The hook at the start of this lesson asked why dams are built much thicker at the base than at the top. Now you know it's because water pressure increases with depth — the base must hold back far more force per square metre!
Explain the dam design using pressure, then connect it to why stiletto heels hurt more than an elephant's foot. Use the words pressure, force, and area at least once each.
Q1. Explain why a person wearing stiletto heels causes more damage to a soft floor than an elephant, even though the elephant is much heavier. (3 marks)
Q2. Explain why dams are built much thicker at the base than at the top. (3 marks)
Q3. A tent peg is hammered into the ground. Explain why a pointed tent peg works better than a flat-ended one, using the concept of pressure. Include a simple diagram in your explanation. (4 marks)
Answers
▾MCQ 1
B — Pressure = force divided by area (conceptually). More force concentrated on a small area = greater pressure.
MCQ 2
B — Same 500 N force, but acting on only 0.01 m² (compared to 1 m²). The force is concentrated into 1/100th of the area → 100× more pressure. The large-force options spread over huge areas result in very low pressures.
MCQ 3
C — Snowshoes do not reduce your weight (force). They increase the area your weight is spread over, which reduces the pressure on the snow below. Less pressure = snow doesn't collapse.
MCQ 4
B — Deeper in a fluid, more fluid is above pushing down. More weight above = greater pressure. That's why deep-sea submarines need thick pressure hulls and dams are thicker at the base.
MCQ 5
B — The sharp knife has a very thin edge, creating a tiny contact area. Same downward force, tiny area = very high pressure at the cutting edge. The blunt knife spreads the same force over a larger area = lower pressure = doesn't cut.
Short Answer 1
Model answer: Pressure depends on both force AND area (1 mark). The elephant has more weight (force), but its large flat feet spread that force over a huge area, creating moderate pressure (1 mark). The stiletto heel is a tiny contact area (less than 1 cm²). Even though the person is lighter, the force is concentrated into such a tiny area that the pressure is enormous — enough to dent or damage a soft floor (1 mark).
Short Answer 2
Model answer: Water pressure increases with depth — the deeper the water, the greater the weight of water pressing outward on the dam walls (1 mark). At the base of the dam the pressure is greatest (1 mark), so the dam wall must be thickest there to withstand the force (1 mark).
Short Answer 3
Model answer: The pointed tent peg has a very small contact area at its tip (1 mark). When hammered with the same force, all that force is concentrated on the small pointed tip (1 mark), creating very high pressure at the tip — which drives it into the ground easily (1 mark). A flat-ended peg spreads the same force over a larger area → much lower pressure → doesn't penetrate the ground. Diagram should show two pegs, label the tip area, and use arrows to show force applied (1 mark).