# Kinetics of Motion in Theory of Machines

## Kinetics of Motion:

Kinetics of Motion deals with the motion which takes into consideration the forces or other factors, e.g mass or weight of the bodies. The force and motion are governed by the three laws of motion.

## Newton’s Laws of Motion:

Newton has formulated three laws of motion, which are the basic postulates or assumptions on which the whole system of kinetics as the results, so obtained, agree with the actual observations. These three laws of motion are as follows:

## 1. Newton’s First Law of Motion:

It states that “Everybody continues in its state of rest or of uniform motion in a straight line unless acted upon by some external force”. This is also known as the Law of Inertia.

## 2. Newton’s Second Law of Motion:

It states that “The rate of change of momentum is directly proportional to the impressed force and takes place in the same direction in which the force acts”.

## 3. Newton’s Third Law of Motion:

It states that “To every action, there is always an equal and opposite reaction”.

## Mass and Weight:

Sometimes much confusion and misunderstanding are created, while using the various systems of units in the measurement of force and mass. This happens because of the lack of a clear understanding of the difference between mass and weight. The following definitions of mass and weight should be clearly understood:

1. Mass: It is the amount of matter contained in a given body, and does not vary with the change in its position on the earth’s surface. The mass of a body is measured by direct comparison with a standard mass by using a lever balance.

2. Weight: It is the amount of pull, that the earth exerts upon a given body. Since the pull varies with the distance of the body from the center of the earth, therefore the weight of the body will vary with its position on the earth’s surface.

Note: The confusion in the units of mass and weight is eliminated, to great extent, in SI. units. In this system, the mass is taken in kg and force in newtons. The relation between the mass (m) and the weight (W) of a body is –

W = m.g

m = W/g

Where W is in newtons, m is in kg and g is the acceleration due to gravity.