Energy of a body in Simple harmonic motion Assignment Help

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In general the total energy of a harmonic oscillator consists of two parts,  potential energy (P.E) and kinetic energy (K.E.), the former being due to its displacement from the mean position and latter due to its velocity. Since the position and velocity of the harmonic oscillator are continuously changing, P.E. and K.E. also change but their sum i.e., the total energy (T.E) must have the same value at all times.

(i) Potential Energy: The simple restoring force acting on the harmonic oscillator is given by

617_Energy of a body in Simple harmonic motion.png

Now if the oscillator is displaced through a further displacement dx opposite the force, work done in displacing the object is given by

2362_Energy of a body in Simple harmonic motion1.png

Hence the net work done in displacing the object from mean position (x=0) to (x=x) is given by

871_Energy of a body in Simple harmonic motion2.png

By convention, P.E. at the mean position is given as zero. Hence, above relation gives the magnitude of P.E. of harmonic oscillator at a distance x from the mean position i.e.,

2327_Energy of a body in Simple harmonic motion3.png              . . . (i)

This shows the P.E. is proportional to the square of the displacement and graph showing the variation of potential energy with the displacement will be a parabola given by continuous lines in the figure. P.E. is maximum at maximum distance and is given by

314_Energy of a body in Simple harmonic motion4.png

(ii) Kinetic Energy: Speed of harmonic oscillator is given by equation as

        1005_Energy of a body in Simple harmonic motion5.png

Hence kinetic power of the oscillator is provided by

     1359_Energy of a body in Simple harmonic motion6.png                     . . . (ii)

The graph showing the variation of K.E. with x is shown in figure by dotted lines.

The kinetic energy is biggest when x = 0. Thus

         1589_Energy of a body in Simple harmonic motion8.png      

Now net energy E of the oscillator for distance x is given by

2467_Energy of a body in Simple harmonic motion7.png

           1515_Energy of a body in Simple harmonic motion9.png              (iii)

Thus total energy is independent of the distance. It has constant throughout the motion of the oscillator. Also the net energy is same to maximum value of either K.E. or P.E.

 (iii) Average Value of P.E.  and K.E.: By equation (i) P.E. at distance x is given by

         2147_Energy of a body in Simple harmonic motion10.png

The average value of P.E. for one complete oscillation is given by

          1397_Energy of a body in Simple harmonic motion11.png

Because the average magnitude of sine or of cosine function for the complete cycle is equal to zero.

Now K.E. at x is given by

   1941_Energy of a body in Simple harmonic motion12.png

The average value of K.E. for one complete cycle

KEaverage  355_Energy of a body in Simple harmonic motion13.png

690_Energy of a body in Simple harmonic motion14.png

Thus average values of P.E. and K.E. of harmonic oscillator are equal and each is equal to one fourth of the total energy. 

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