Inertia Theory

Physics is the study of nature. Its goal is to explain the basic principles underlying various phenomena we observe in nature through two distinct mechanisms: theory and experiment. A theory is based on predictions that relate various quantities needed to explain a phenomenon, and its validity is tested in a laboratory through experiments. If there is discrepancy between the two results, the theory is considered incorrect, and we resort to looking for a more accurate theory. This itself speaks for the importance of precise measurements in experiments. The result of an experiment is recorded as the value of the physical quantity, which is expressed as a number followed by a unit. For example, the height of a person is measured and recorded as

the state of rest or the state of motion. In physics, we say that all bodies possess a certain built-in property called inertia which is defined as its reluctance to change its state. That means a body, which is at rest, does not want to move and a body, which is already moving, does not want to stop moving and come to rest unless a force acts on it. The question is; what property of the body is responsible for its inertia? We say that it is the inertial mass, defined as the measure of inertia of a body. Its unit is kilogram (kg). If we apply an external force on the body, it will change its state by producing acceleration (or deceleration) in it. [See chapter 2 for definition of acceleration]. The more the inertial mass of the body the more the force required to produce the same acceleration in it. We know from our common experience that more force is required to move a heavy vehicle (large mass) than a light one (small mass). In fact from Newton’s second law of motion (chapter two) we can show that the force ‘F’ required to produce an acceleration of ‘a’ in a body of mass ‘m’ is equal to ‘m’ times ‘a’. That is F = ma. When referring to the force applied by the earth on a body, the equation is F = mg, where g is called the acceleration due to gravity and the force is called the weight of the body. In this context, the mass is referred to as the gravitational mass. (Details in chapter 2). The unit of weight is newton (N). It can be shown that the weight of a body is proportional to its mass. This fact is made use in measuring the mass of a body by comparing its weight with a known weight in a beam balance or a lever balance as discussed in the following section. It should be noted that weight of a body is not the same as its mass, which has nothing to do with gravity. Therefore, mass of a body is the same everywhere, but weight differs from place to place because the force of gravity is