 What is sound? Oh my god. Once we understand, so in your lecture we will do sound check around. So, please write down wave under the sound like this. What comes in your mind? Any example that comes in your mind, the sound picture? C wave. C wave, sit down. Then what? Face is up. What? Face is up. Face is up. Okay, fine. Nice. Then what is? Are you a guitarist? No, sir. So I have to show you about it. Very good, right? Very good. Who is? Sir, the sound. What? Saw two waves. Saw two waves. Don't care about it. Then, motion of study of mass. In Russia, mass follows votes to mass immigration. Mass has momentum. And if mass is sterile with constant acceleration you can use, equations from motion have not studied the mass. We are studying which is the kind of disturbance difference which travels from one point to the other point. Which travels from one point to the other point. Okay? The examples could be as simple as a wave traveling on a string. Okay? We can visualize this in your head that you have created a ripple by moving this and up and down, then this ripple will move forward. Okay? Similarly, water ripples, if you throw stone on the water, the waves get generated. And do you think it seems as if water is residing. It is generated but in forward. But water is there only. Then what moves forward? Something moves forward. And that is the way. Similarly, most of the medium, a means to go forward. This medium could be solid also. Fine? But there are many waves which do not require signals from this point to that point. How to use waves? When I am spending that energy, there can be moves forward. It moves forward. Stone, rocket. So, examples? Forward. It moves forward. When I move the wave.