All quantities other than length, distance, mass and time are called secondary or derived quantities of mechanics.
Speed,velocity,acceleration,weight,force,moment, momentum etc. are secondary quantities,
All these quantities can be expressed in terms of fundamental quantities. For example acceleration
can be expressed in terms of length/unit time/ unit time [L T-2 ].
Some secondary quantities are described below:
（i) Displacement: The directional distance between initial and final positions of the body is
called displacement. It is a vector quantity and its unit in the SI system is metre.
In order to clearly understand the difference between distance and displacement, we may consider
the example of a ball projected upward to a height of 3 m and returns to the point of projection. In
this case, the total distance covered by the ball is 6 m while the displacement of the ball is zero.
（ii) Speed and velocity: The distance travelled by a body per unit time without any reference
to the direction of motion is called speed. It is a scalar quantity. It is denoted by ‘S’ and its unit is
m s-1. The distance travelled by a body per unit time in a particular direction is called velocity. It
is a vector quantity. It is denoted by ‘V’ and its unit is also m s-1.
（iii) Acceleration: The rate of change of velocity of a body is called acceleration. It is denoted by ‘a’ and its unit is m s-2. It is a vector quantity. The acceleration of the body will be
uniform when its velocity changes by an equal amount in equal intervals of time and is variable when
its velocity changes by unequal amounts in equal intervals of time.
（iv) Force: Force may be defined as an action (push or pull) which changes or tends to
change the state of rest or of uniform motion of a body. This definition of force follows from
Newton’s first law of motion. Its unit in the SI system is the newton.
This definition applies to the external effects of a force. The internal effect of a force is to
produce stress and deformation in the body. External effects of forces are considered in engineering
mechanics; Internal effects, in the strength of materials.
（v) Weight: The weight of a body is the force with which it is being attracted towards the
centre of the earth. The weight of the body is proportional to its mass i.e., the quantity of matter in
the body.
Since the value of acceleration due to gravity varies from place to place so the weight of a
body is not a constant quantity. A body weighs more at the poles than at the equator. Again a body
weighs less on a mountain than in the plains. The weight of the body,6400 x 103 metres above the
earth’s surface will be reduced to 1/4th of what it is on the surface of the earth.
（vi) Density: It may be defined as mass per unit volume. It is denoted by p (rho).In SI system
the unit of density is kg m-3.