Basic Terminologies in Mechanics

1) Mass (m)

The quantity of the matter possessed by a body is called mass. The mass of a body will not change unless the body is damaged and part of it is physically separated. If the body is taken out in a space craft, the mass will not change but its weight may change due to the change in gravitational force. The body may even become weightless when gravitational force vanishes but the mass remain the same.

2) Weight (w)

Weight of a body is the force with which the body is attracted towards the centre of the earth. The weight of the body is equal to the product of mass and the acceleration due to gravity. This quantity of a body varies from place to place on the surface of the earth.

Mathematically,

w=mg

Where ‘w’ is the weight of the body, ‘m’ is the mass of the body and ‘g’ is the acceleration due to gravity.

Table 1 Difference between Mass and Weight

Mass	Weight
Mass is the total quantity of matter contained in a body.	Weight of a body is the force with which the body is attracted towards the centre of the earth.
Mass is a scalar quantity, because it has only magnitude and no direction.	Weight is a vector quantity, because it has both magnitude and direction.
Mass of a body remains the same at all places. Mass of a body will be the same whether the body is taken to the centre of the earth or to the moon.	Weight of body varies from place to place due to variation of ‘g’ (i.e., acceleration due to gravity.
Mass resists motion in a body.	Weight produces motion in a body.
Mass of a body can never be zero.	Weight of a body can be zero.
Using an ordinary balance (beam balance), the mass can be determined.	Using a spring balance, the weight of the body can be measured.
The SI unit of the mass is the kilogram (kg).	The SI unit of the weight is Newton (N).

3) Time

The time is the measure of succession of events. The successive event selected is the rotation of earth about its own axis and this is called a day. To have convenient units for various activities, a day is divided into 24 hours, an hour into 60 minutes and a minute into 60 seconds. Clocks are the instruments developed to measure time. To overcome difficulties due to irregularities in the earth’s rotation, the unit of time is taken as second, which is defined as the duration of 9192631770 period of radiation of the cesium-133 atom.

4) Space

The geometric region in which study of body is involved is called space. A point in the space may be referred with respect to a predetermined point by a set of linear and angular measurements. The reference point is called the origin and the set of measurements as coordinates. If the coordinates involved are only in mutually perpendicular directions, they are known as cartesian coordination. If the coordinates involve angles as well as the distances, it is termed as Polar Coordinate System.

5) Length

It is a concept to measure linear distances. Meter is the unit of length. However depending upon the sizes involved micro, milli or kilo meter units are used for measurements. A meter is defined as length of the standard bar of platinum-iridium kept at the International Bureau of weights and measures. To overcome the difficulties of accessibility and reproduction now meter is defined as 1690763.73 wavelength of krypton-86 atom.

5) Continuum

A body consists of several matters. It is a well known fact that each particle can be subdivided into molecules, atoms and electrons. It is not possible to solve any engineering problem by treating a body as conglomeration of such discrete particles. The body is assumed to be a continuous distribution of matter. In other words the body is treated as continuum.

6) Particle

A particle may be defined as an object which has only mass and no size. Theoretically speaking, such a body cannot exist. However in dealing with problems involving distances considerably larger compared to the size of the body, the body may be treated as a particle, without sacrificing accuracy.

For example:

• A bomber aeroplane is a particle for a gunner operating from the ground.
• A ship in mid sea is a particle in the study of its relative motion from a control tower.
• In the study of movement of the earth in celestial sphere, earth is treated as a particle.

7) Rigid Body

A body is said to be rigid, if the relative positions of any two particles do not change under the action of the forces acting on it i.e., the distances between different points of the body remain constant. No body is perfectly rigid. Rigid body is ideal body.

Fig. 1 Rigid Body due to the action force F

8) Deformable Body

When a body deforms due to a force or a torque it is said deformable body. Material generates stresses against deformation. All bodies are more or less elastic.

Fig. 2 Deformable Body due to the action force F