Suppose you have to fix a poster on your class bulletin board, you have to apply force on the head of the drawing pin, i.e., the force is perpendicular to the surface of the bulletin board and this force which is acting perpendicular to the surface is called thrust.
Thrust is the force acting on a body perpendicular to its surface.
S.I unit of thrust is Newton (N).
Now let us see whether there is a relation between the force applied (thrust) and the area on which it is acting.Activity I
Hold a pin erect on a pile of papers. Keep another pin next to it upside down in such a way that its flat head rests on the pile. Press both these pins down by placing a flat object such as duster. We observe that pin which is erect pierces through the pile of papers.
Activity II
Hold your bag with a strap made of a thin but strong string. Now lift the same bag with another strap made of a wide cloth band. You will find that carrying a school bag with a wide cloth band is more comfortable than carrying the same with a thin strip. This is because in the second case the weight of the books is distributed over a large area of the shoulder exerting less force.Thus from the above examples it is clear that the effectiveness of the force applied depends on the area on which it is acting.
Now there arises a need to define a new physical quantity called pressure.Pressure is defined as the force acting on a unit area.
S.I unit of pressure is N/m2. N/m2 is known as Pascal (Pa) in honour of the French Scientist Blaise Pascal.
1 N/m2 = 1 PascalPascal is a very small unit and hence often we use kilopascal. Pressure depends on two factors namely, force applied and area over which force acts.
