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E1.5 Work - Energy Theorem:

Net work on an object causes a change  in the kinetic energyThe work done in a body due to a force is equal to the change in its kinetic energy. 
i.e. Work done = Change in Kinetic Energy
Consider a body of mass M, moving with a initial velocity u  when a constant force F is acts on it. Let final velocity of the body becomes v after covering a distance d in a direction of force. Then, work done by the force is given by
W=F.d .......... (i)
From the Newton's law of Motion:
F=ma .......... (ii)
From equation (i) and (ii)
F=ma.d .......... (iii)
And from the Kinematics, we know the formula:
v2=u2+2ad
d=v2u22a .......... (iv)
From the equation (i), (iii) and (iv), we get
W=ma(v2u22a)
      =m(v22u22)
     =12mv212mu2
      = Final K.E - Initial K.E.
∴ Work done = Change in Kinetic Energy
So, the work done on moving the body by the force is equal to the increase in its kinetic energy.
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