NCERT 9TH CLASS SCIENCE CHAPTER WORK AND ENERGY

WORK

    The intuitive meaning of work is quite different from the scientific definition of work. In everyday activity, the term 'work' is used equally for mental work and for physical work (involving muscular force) as is clear from the following examples. 
(i)    You may read a book or exert yourself mentally in thinking about a simple or difficult problem. 
(ii)    You might be holding a weight without moving. 
(iii)    You may be carrying a load and moving with uniform velocity. 
(iv)    You may be trying hard to move a huge rock which does not move despite your best efforts, though you may get completely exhausted in the process. 
    In all these cases, according to scientific definition, you are not doing any work. 

SCIENTIFIC CONCEPTION OF WORK 

    In physics, the term work is used in a special technical sense and has a much more precise definition which follows from the following examples. 
(i)    When a box is pushed on a floor by applying a force and it moves through some distance, work is said to be done. In this case, the applied force displaces the box. 
(ii)    When we pull a trolley by applying a force and it moves through some distance, work is again said to be done. 
(iii)    When we lift a box through a height, we have to apply force. In this case, the applied force does work in lifting the box. 
    From all the examples given above, it follows that work is done if : 
    (a) a force is applied on the object and 
    (b) the object is displaced from its original position. 
    No work is said to be done if any of the two conditions is not satisfied. 
    

 

UNITS OF WORK DONE 

    Work done, W = Fd
    In C.G.S. system the unit of work done is dyne x cm = erg.
    Definition of 1 erg:
    If     F = 1 dyne and d = 1 cm
    then,     W = 1 × 1 = 1 erg.
    If one dyne force is applied on a body and displacement in the body is 1 cm in the direction of force, then work done will be one erg.
    S.I. unit of work done is newton × metre = joule
    Definition of 1 joule:
    If    F = 1N and d = 1m
    then,     W = 1 × 1 = 1 joule (J)
    If a force 1 Newton is applied on a body and displacement in the body is 1m in the direction of force then work done will be 1 joule.
    Relation between joule and erg:
    1 joule = 107 erg

    Erg and joule are the absolute units of work done.
   

 CONCLUSION

1.    When s = 0, W = 0, i.e. work done by a force on a body is zero 
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