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# Which of the Following Functions of Time Represent (A) Simple Harmonic, (B) Periodic but Not Simple Harmonic, and (C) Non-periodic Motion? Give Period for Each Case of Periodic Motion (ω is Any Positive Constant) - CBSE (Science) Class 11 - Physics

ConceptSimple Harmonic Motion and Uniform Circular Motion

#### Question

Which of the following functions of time represent (a) simple harmonic, (b) periodic but not simple harmonic, and (c) non-periodic motion? Give period for each case of periodic motion (ω is any positive constant):

a) sin ω– cos ωt

(b) sin3 ωt

(c) 3 cos (π/4 – 2ωt)

(d) cos ω+ cos 3ωt + cos 5ωt

(e) exp (–ω2t2)

(f) 1 + ωω2t2

#### Solution 1

a) SHM

The given function is:

sin omegat - cos omegat

= sqrt2[1/sqrt2 sin omegat - 1/sqrt2 cos omegat]

=sqrt2[sin omegat xx cos pi/4 - cos omegat xx sin pi/4]

= sqrt2 sin (omegat - pi/4)

This function represents SHM as it can be written in the form: asin (omegat  + phi)

Its period is : (2pi)/omega

b) Periodic, but not SHM

The given function is:

sin3ωt=143sinωt-sin3ωt

The terms sin ωand sin ωt individually represent simple harmonic motion (SHM). However, the superposition of two SHM is periodic and not simple harmonic.

c) SHM

The given function is:

3cos[pi/4 - 2omegat]

= 3cos[2omegat - pi/4]

This function represents simple harmonic motion because it can be written in the form: acos(omegat + phi)

Its period is (2pi)/(2omega) = pi/omega

d)Periodic, but not SHM

The given function is cos omegat + cos 3omegat + cos 5omegat Each individual cosine function represents SHM. However, the superposition of three simple harmonic motions is periodic, but not simple harmonic.

e) Non-periodic motion

The given function  exp(-omega^2r^2) is an exponential function. Exponential functions do not repeat themselves. Therefore, it is a non-periodic motion.

f) The given function 1 + ωt + ω2t2 is non-periodic.

#### Solution 2

The function will represent a periodic motion, if it is identically repeated after a fixed interval of time and will represent S.H.M if it can be written uniquely in the form of a cos

((2pit)/T + phi) or a sin ((2pit)/T + phi) , where T is the time period

a) sin omegat  - cos omegat =  sqrt2[1/sqrt2 sin omegat - 1/sqrt2 cos omegat]

= sqrt2[sin omegat cos  pi/4 - cos omegat sin  pi/4]

= sqrt2 sin (omegat  - pi/4)

It is a S.H.M and its Period is 2 pi"/"omega

b) sin^3 omegat = 1/3 [3sin omegat - sin 3omegat]

Here each term sin omegat and sin 3 omegat individually represent S.H.M. But ii which is the outcome of the superposition of two S.H.M will only be periodic but not SHMs..

Its time period is 2pi"/"omega

c) 3 cos (pi/4 - 2omegat) = 3 cos(2omegat - pi/4)      [∵ cos(-theta) = cos theta]

Clearly it represent SHM and its time period is 2pi"/"2omega

d) cos omegat + cos 3 omegat  + cos 5 omegat. It represent the periodic but not S.H.M. Its time period is 2pi"/"omega

e)e-w2t2 . It is an exponential function which never repeats itself. Therefore it represents non-periodic motion

f) 1 + wt + w2t2 also represents non periodic motion.

Is there an error in this question or solution?

#### APPEARS IN

Solution Which of the Following Functions of Time Represent (A) Simple Harmonic, (B) Periodic but Not Simple Harmonic, and (C) Non-periodic Motion? Give Period for Each Case of Periodic Motion (ω is Any Positive Constant) Concept: Simple Harmonic Motion and Uniform Circular Motion.
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