All flowing substances – both liquids and gases – are called fluids. A small iron nail sinks in water, whereas a huge ship floats on sea water; astronauts have to wear a special suit in space. All these have a common reason: pressure. If pressure increases on a solid, it experiences tension and ultimately deforms or breaks; in fluids, pressure causes them to flow rather than deform. A gas is easy to compress, whereas liquids are incompressible.
1. Thrust and Pressure
Thrust
- The force acting perpendicular to a surface is called thrust. It is the force which produces compression.
- SI unit of thrust: newton (N).
- The effect of thrust depends on the area on which it acts. When you stand on loose sand, your weight acts on the area of your feet and you sink deep; when you lie down, the same weight acts on the larger area of your whole body and you do not sink as deep.
Pressure
- The force per unit area acting on an object is called pressure. Thrust on a unit area is pressure.
- Pressure = Thrust / Area of contact, i.e. P = F / A.
- For the same force, if the area is large, pressure is low; if the area is small, pressure is high.
- People can lie on a bed of nails and remain unhurt because the area of contact is more.