Force spread over an area
Why can you walk on snow in snowshoes but sink in boots? Why does a sharp knife cut better than a dull one? The answer is pressure: how concentrated a force is.
pressure = force ÷ area
P = F ÷ A
| Quantity | Unit |
|---|---|
| Pressure | Pascal (Pa), 1 Pa = 1 N/m² |
| Force | Newton (N) |
| Area | Square metre (m²) |
A pascal is tiny, about the pressure of a sheet of paper resting on a table. We often use kilopascals (kPa).
Big force or small area
- Same force, smaller area → more pressure.
- Same force, bigger area → less pressure.
| Small area, high pressure | Large area, low pressure |
|---|---|
| Knife edge cuts easily | Snowshoes keep you on top of snow |
| Nail point pierces wood | Wide tractor tires don’t sink in mud |
| Skate blade melts a thin film of ice | Camels’ wide feet on sand |
| High heels dent soft floors | Lying flat on thin ice spreads your weight |
Example
A 600 N person stands on two feet with a total area of 0.04 m².
P = 600 ÷ 0.04 = 15 000 Pa (15 kPa)
On snowshoes with an area of 0.3 m²: P = 600 ÷ 0.3 = 2000 Pa, much less, so they don’t sink.
Pressure in liquids
Liquids push in all directions. Pressure in a liquid increases with depth, because there’s more liquid above pressing down.
- Every 10 m deeper in water adds about 100 kPa, roughly the pressure of the whole atmosphere.
- Dams are thicker at the bottom, where water pressure is greatest.
- Divers’ ears hurt as they go deeper.
- Submarines need thick, strong hulls. At the bottom of the Mariana Trench, about 11 km down, pressure is over 1000 times that at the surface.
Hydraulic systems
Liquids can’t be squeezed into a smaller space. A hydraulic system uses this to multiply force: pushing on a small piston creates pressure that is passed through the liquid to a large piston, which pushes with a much bigger force.
Uses: car brakes, car lifts in garages, excavators, dentist chairs and airplane controls.
Pressure in gases
Atmospheric pressure
Air has weight. The air above you presses down with atmospheric pressure, about 101 kPa at sea level. That’s like having a mass of about 10 tonnes pressing on every square metre! You don’t feel it because the fluids inside your body push outward equally.
At higher altitudes, there’s less air above, so pressure is lower.
| Location | Approximate air pressure |
|---|---|
| Sea level | 101 kPa |
| Calgary (about 1050 m) | 89 kPa |
| Top of Mount Everest (8849 m) | About 34 kPa |
That’s why your ears pop in airplanes and in mountains, and why airplane cabins are pressurized.
Measuring air pressure
A barometer measures atmospheric pressure. Weather forecasters use it:
- Rising pressure usually means fair weather.
- Falling pressure often means clouds, rain or storms.
Gas pressure in containers
Gas particles constantly hit the walls of their container, creating pressure.
- Squeeze a gas into a smaller space → more collisions → higher pressure (bike pump).
- Heat a gas → particles move faster → higher pressure (why aerosol cans shouldn’t be heated).
Using air pressure
- Drinking straws: you lower the pressure in the straw; atmospheric pressure pushes the drink up.
- Suction cups: pushing out the air leaves low pressure inside; outside air holds the cup on.
- Vacuum cleaners: a fan lowers pressure inside, and outside air rushes in carrying dirt.
Buoyancy
Pressure is also behind buoyancy. Water pushes up harder on the bottom of an object than it pushes down on its top, because pressure increases with depth. This upward push is the buoyant force. An object floats if the buoyant force is at least equal to its weight. Archimedes discovered this principle more than 2200 years ago.
Common mistakes
- Mixing up force and pressure. The same force can create very different pressures.
- Thinking air has no weight.
- Thinking water pressure only pushes down. It pushes in all directions.
Try it yourself
- Press a pencil between your palms, one hand on the point and one on the eraser. Which hand feels more pressure? Why?
- Poke three holes at different heights in a plastic bottle full of water. Which stream shoots out farthest?
- Fill a glass to the brim, place a card on top, and carefully turn it over (over a sink). Why doesn’t the water fall?
Quick recap
- Pressure = force ÷ area, measured in pascals.
- Smaller area means more pressure; larger area means less.
- Liquid pressure increases with depth and acts in all directions.
- Atmospheric pressure decreases with altitude and helps forecast weather.
- Hydraulic systems use liquid pressure to multiply force.