Some bodies like table, desk, bucket full of water press the floor with a force equal to their own weight. The weight is the force acting vertically downwards. As the surface of the floor can be taken as horizontal, therefore, the force with which each body presses the floor is directed perpendicular to the surface of the floor.
The force acting upon the surface of a body perpendicular to it is called thrust.
The action of the given thrust depends on the area of the surface it acts upon. The smaller the area on which the thrust acts, the more evident is the result of its action. The thrust on unit area is called pressure.
Pressure = Thrust / Area
The SI unit of pressure is Nm–2. This unit has been given a specific name pascal (Pa) in honour of the scientist named Blaise Pascal.
Equation shows that the same force acting on a smaller area exerts a larger pressure and a smaller pressure on a larger area. This is the reason why cutting tools like knives and axes always have sharp edges.
In many cases it is desirable to decrease pressure. In such cases the area on which the thrust is acting should be increased. For example, foundation of buildings and dams are made on larger area. Similarly trucks and vehicles used to carry heavy loads have much wider tyres. Also, army tank weighing more than a thousand tonne rests upon a continuous chain.