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3.2.1 Concept of Work in Mechanics

Work is done by a force when the application of force on a body moves the body in the

direction of force. Work in measured by the product of the force and the distance moved

in the direction of force.


The work , W = F.d

Where                          f=  force, d =displacement


The S.l unit of work     = N.m or jouls

1Nm = 1J.

The action of a force through a distance (or of a torque through an angle) is called mechnnicnl work.


3.2.2 Thermodynamics Concept of Work


Work is performed when the system changes its state i.e., a process is done. In many thermodynamic process, the force and / or the  displacement may not be visible. The definition of work needs re-defining for thermal system. W work is said to be done by a system if the sole effect on things external to the system can be reduced to the raising of a mass against gravity. The weight (mg) may not actually be raised, but the net effect external to the system would be the raising of a weight.

Fig. 3.2 (a) shows a motor driving a fan. The system is doing work W on the surroundings (jan). In Fig. 3.2 (b) the fan is replaced by a pulley and a weight. The weight may be raised by the pulley driven by the motor.

The sole effect on things external to the system is nothing but raising the weight.


3.2.3 Conveniton of sigh of work

The work is positive when it is done by the system. The work is negative when it is done on a system.

Fig. 3.3 (a) shows positive work whereas Fig. 3.3 (b) shows negative work. Power is defined as the rate at which work is done by, or Fig. 3.3 upon, the system. Thus the unit of power is 1  J /S Ys or watt. The system and surroundings can interact with each other through this form of energy i.e., work. The unit of power is also expressed as hp.

1 hP = 746 watt (British)

1 metric hp = 745.5 watt.