**1.** **Mass**

It is the quantity of matter possessed by a body. The SI unit of mass is the kilogram (kg). The mass of a body is a constant quantity and is independent of the place and position of the body. Thus the mass of a body is the same whether it is on the earth’s surface, the moon’s surface, on the top of a mountain, or down a deep well.

**2. Force**

It is the product of mass (kg) and acceleration (m/s^{2}). The unit of force is newton (N); being the force required to accelerate a mass of 1 kg through an acceleration of 1 m/s^{2}.

F = m a newtons

where m = mass of the body in kg

a = acceleration in m/s^{2}

**3. Weight**

The force with which a body is attracted towards the center of the earth is called the weight of the body. Now, force = mass x acceleration. If it is the mass of a body in kg and g is the acceleration due to gravity in m/s^{2}, then,

Weight, W = m g newtons

As the value of g varies from place to place on the earth’s surface, therefore, the weight of the body varies accordingly. However, for practical purposes, we take g = 9.81 m/s^{2} so that the weight of the body = 9.81 m newtons. Thus if a mass of 1 kg rests on a table, the downward force on the table i.e., the weight of the body is W = 9.81 x 1 = 9.81 newtons.

**4. Electrical Energy**

The SI unit of electrical work done or electrical energy expended in a circuit is also joule โexactly the same as for mechanical energy. It is defined as under :

One joule of energy is expended electrically when one coulomb is moved through a p.d. of 1volt.

**5. Efficiency**

The efficiency of a device or of a circuit is the ratio of useful output to the. input i.e.

Efficiency, ฮท = useful output/input = W_{o}/W_{i}

The law of conservation of energy states that “energy cannot be created or destroyed but can is converted from one form to another-. Some of the energy in electric circuits may be converted into a form that is not useful. For example, consider an electric motor shown in Fig. 1. The purpose of the motor is to convert electric energy into mechanical energy. It does this, but it also converts a part of input energy into heat. The heat produced is not useful. Therefore, the useful output energy is less than the input energy. In other words, the efficiency of the motor is less than 100%.

Some electrical devices are nearly 100% efficient. An electric heater is an example. in a heater, practically all the input electric energy is converted into heat energy. In this case, the heat is useful to output.

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