Simple Harmonic Motion,as the name implies, is any motion that can be described by a simple harmonic function.But,that is too simple a definition for beginners like you to get anything.I am not going to tell you what are harmonic functions.That's a bit too deep into calculus.But I am going to give you examples of such functions and they are functions very close to our hearts, sines and cosines.

Please give some examples of such a motion.

sines and cosines are periodic function.Does that mean SHM is also periodic in nature?

You have raised a good point Rahul.SHM is indeed a type of periodic motion.As for you Rohina,examples of such motions are familiar gravitational pendulum and spring-mass system.In all these examples as you must have probably observed there is a restoring force that acts in direction opposite to displacement.As the name implies this force tends to restore the least energy state in the system.Remember I told you every system tends to seek the least potential energy configuration.

Sir,I did not get a feel.I mean the idea did not sink in my head.Would you please elaborate?

Ok Rahul,lets start with the example I mentioned earlier,the spring mass example.Let me ask you a simple question,what force would act on a block connected with a spring when given a small displacement x.

Sir,the force would be kx.

Oh! Rohina. Rahul was facing problem in getting the concept. You should have allowed him to answer. Anyways, your answer is not totally correct. The direction of this force is important. It opposes the rate of change of velocity. Hence the right answer is -kx. As I told you this opposes the displacement. It would try to bring the mass in the initial position.

hmm...mm but sir once the block has come to the initial position ,why should it move further. At mean the spring force would be zero.

Inertia ,the block would be moving due to inertia .Let me show you the situation from energy point.When you have extended the spring by pulling the block,it has gained potential energy (0.5*k*x*x).But I know that this spring-mass system or for that matter any system in this universe tends to seek the least energy configuration .So what does the spring do??? It pulls the block toward the mean position so that the system could be in the preferred position. But as in life so in physics the things that we prefer or wish for seldom happens. As soon as the spring reaches it's preferred position the block ruthlessly crushes all its dream by pushing it from that position. But the spring never stops trying.It continues to bring the system to the least energy position but the block also continues to move due to inertia and thanks to this pandemonium we have a chapter known as SHM and I my livelihood. Is that clear now?

Yes Sir,now the picture is now clear in my mind.

Thats good,now let me wind up the lecture by giving you the mathematical details and some technical definition.You can see the mathematical details in the above two demo.As for terminology lets go one by one.