Extension: Pressure, Qualitative
You'll be able to predict which situation produces more pressure given the same force.
Engineers designing the 2023 Snowy 2.0 tunnel had to work out how hard the rock above presses on every square metre of concrete lining, and the answer changes with depth along the same tunnel.
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, and the reason area matters becomes obvious if you picture the force being shared out. Push on that drawing pin with 10 N. The flat head is about a centimetre across, so the 10 N is shared among a whole disc of skin under your thumb and each small patch of it takes almost nothing. The point at the other end is a fraction of a millimetre across, and the very same 10 N now has to pass through that one tiny patch of wall. Nothing about the force changed between the two ends of the pin. What changed is how many patches were available to share it out among.
That gives you both halves of the rule and the reason for each. A smaller area increases the pressure, because the same force is forced through fewer places at once. A larger area reduces the pressure, because it is spread among more of them. It also explains something that force on its own cannot: why a knife cuts. A knife does not push harder than your hand does, since it is your hand doing the pushing. It pushes through less.
Butter on toast makes the same point in one movement. Press with the flat of the knife and the butter barely moves. Turn the same knife onto its edge, press with exactly the same hand and the same effort, and it cuts straight through. One variable changed, and it was not the force.
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?
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).
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.
- Pressure is how concentrated a force is: the same force on a smaller area produces more pressure, and on a larger area produces less.
- This is why a sharp knife cuts and a blunt one does not, why snowshoes and camel hooves stop you sinking, and why a stiletto heel can press harder on a floor than an elephant's foot.
- In a fluid, pressure increases with depth, because there is more fluid above pressing down. That is why dam walls and submarine hulls are built strongest at the bottom.