Force, pressure, and head

FORCE, PRESSURE, AND HEAD

1. Force

• Definition : Force may be defined as an agent, which produces or tends to produce, destroy or tends to destroy motion.

• Formula: According to Newton's second law of motion, the applied force is directly proportional to the rate of change of momentum. Mathematically,

• Units : newtons (N)

• Dimensions: MLT-2

2. Pressure

• Concept: Pressurized fluid is used in the hydraulic systems to transform the energy and consequently to produce linear or rotary motion. Pressure occurs in a fluid when it is subjected to a force.

• Definition : Pressure may be defined as force per unit area.

• Formula: In Fig.3.1, a force F is applied to an enclosed fluid via a piston of area A.

Then,

• Units: newtons per square metre (N/m2). 1 Pascal = 1 N/m2. [Note that the most frequently used alternative SI unit is the bar; 1 bar = 105 N/m2]

• Dimensions : ML-1T-2

3. Pressure Head

• The law of hydrostatics states that the pressure P of a fluid at rest increases on increasing the depth. It is also called as head pressure.

• Formula: Consider the four vessels having the same base area A and are filled to the same height.h with the same liquid of density p, as shown in Fig.3.2.

• In Fig.3.2, points A, B, C and D are at equal depths in water and therefore have identical pressure. But points a, b, c and d are at different depths and hence they have different pressures.

Since the pressure exerted by a fluid is dependent only on the vertical head of fluid and its mass density p, we can write

Pressure on bottom in each case, P = ρ g h       …... (3.3)

• Thus the intensity of pressure of a liquid may be expressed in the following two ways: 

1. As a force per unit area (i.e., N/m2), and

2. As an equivalent static head (i.e., metres, mm or cm of liquid).