Forces & Motion
Pressure
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓Define pressure and state its SI unit.
- ✓Calculate pressure using the formula P = F/A, ensuring correct unit conversions.
- ✓Explain how pressure in fluids changes with depth and acts in all directions.
- ✓Describe atmospheric pressure and provide examples of its effects in everyday life.
Key concepts
Pressure is defined as the force applied perpendicular to a surface divided by the area over which the force is distributed. It tells us how concentrated a force is. A small force over a tiny area can produce a very large pressure, while a large force spread over a big area might result in a small pressure.
The SI unit for pressure is the Pascal (Pa). One Pascal is defined as one Newton of force applied over one square metre of area (1 Pa = 1 N/m²). Other units sometimes encountered include the kilopascal (kPa), which is 1000 Pa, and the bar, often used in meteorology.
Fluids (liquids and gases) exert pressure. This pressure acts equally in all directions at a given depth. For liquids, pressure increases with depth because there is more fluid above, meaning a greater weight of fluid is pressing down. This is why your ears 'pop' when you dive deeper into water. Gases also exert pressure, but because they are compressible, their pressure changes more significantly with altitude (e.g., atmospheric pressure).
Atmospheric pressure is the pressure exerted by the weight of the air in the Earth's atmosphere. We live at the bottom of an 'ocean of air'. This air has mass, and gravity pulls it down, creating pressure. Atmospheric pressure is quite large at sea level (approximately 101,325 Pa or 101.3 kPa), but we don't usually feel it because it acts equally in all directions, and our bodies are adapted to it. It decreases as you go higher in altitude because there is less air above you.
Key facts to remember
- 1Pressure is defined as force per unit area (P = F/A).
- 2The SI unit of pressure is the Pascal (Pa), where 1 Pa = 1 N/m².
- 3Pressure increases if the applied force increases or if the area over which the force is applied decreases.
- 4In fluids (liquids and gases), pressure acts in all directions.
- 5Pressure in a liquid increases with depth.
- 6Atmospheric pressure is caused by the weight of the air above us.
- 7Examples of atmospheric pressure effects include drinking with a straw and suction cups.
Worked examples
Example 1
A box with a weight of 500 N rests on the ground. If the area of its base is 0.25 m², calculate the pressure the box exerts on the ground.
Answer
The pressure exerted by the box is 2000 Pa.
Remember that weight is a force, measured in Newtons.
Example 2
A student weighing 600 N stands on one foot. The area of the sole of their shoe is 0.015 m². Calculate the pressure exerted on the ground. What would happen to the pressure if they stood on both feet?
Answer
Pressure on one foot = 40,000 Pa. If standing on both feet, the pressure would decrease to 20,000 Pa.
Pressure is inversely proportional to area when force is constant.
Example 3
Explain, in terms of pressure, why a sharp knife cuts more easily than a blunt knife.
Answer
A sharp knife has a very small cutting edge, meaning the force applied is concentrated over a tiny area. According to P = F/A, a smaller area results in a much greater pressure for the same applied force. This high pressure allows the sharp knife to easily cut through materials, whereas a blunt knife, having a larger area, would exert less pressure and be less effective.
Common mistakes
- ✗Confusing force (measured in Newtons) with pressure (measured in Pascals).
- ✗Forgetting to convert units, especially area, from cm² to m² (1 m² = 10,000 cm²).
- ✗Assuming pressure only acts downwards in a fluid; it acts in all directions.
- ✗Not understanding that atmospheric pressure is a significant force that we are constantly under.
- ✗Incorrectly stating that pressure increases with height in a fluid (it decreases).
Exam tips
- ★Always write down the formula (P = F/A) before substituting values.
- ★Pay close attention to units. If area is given in cm², convert it to m² before calculating pressure.
- ★Clearly show all steps in your calculations, including units at each stage.
- ★When explaining concepts, use correct scientific terminology like 'force', 'area', 'pressure', 'fluid', 'depth', and 'weight of air'.
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