 So, what do you think the next will be after semi-circuit disc? Semi? Hemisphere. Hemisphere, hollow hemisphere. Write down. It's a hollow and what do you think DM should be? Surveyor rings, correct. You take a surveyor ring. This angle, sorry. This angle is T theta. Okay. And the radius is R, capital R. Find out the center of mass location. Of course, this is linear surveyor disc. It's a ring. Now, thickness of the ring is making some angle. It's not if the entire plate is at angle. So, we are not getting to solid angle. It is not in yourselves. It's a clear hemisphere one. It is simple. It's straight forward. Again. I'll just write the formula opening. This. Find out DM. Find out YCM of the T. So, Y center of mass is minus of R sine theta. This distance is a sine. So, DM will be mass per meter. 2 pi R is... I don't know how to say all this. Okay. M derivative in the strip and made up. So, 2 pi into small r sine 2 theta. Sorry, 0 to pi by 2. Very sorry about that. Minus cos 0 to pi by 2. So, in a substitute, we are going to Y center of mass to be equal to minus of R by 2. Holo hemispheres. What will be the next one? Solid hemispheres. Solid hemispheres. Center of mass. Now, you are finding center of mass after this. Last one. They are width, right? The small increases here. If you have to plot the entire hemisphere to increase the value of small r, right? You have to find out such things. Which is the last one. That is not the volume of the DM. That is the volume inside the DM you have found out. See, it is like volume of the material of this. It is not the volume of this. It is not the total volume. So, the volume of the material is the surface area into the thickness. Total volume is the volume of what I am saying. But that is not the volume of the. Similarly, volume of DM is surface area of the DM into the thickness. Okay. Y cm is not. What? Y cm is minus of r2 by 3 inside DM. I am finding volume of the two different sides. Okay. Pi and pi gone. So, I am getting 3m by rq r square DM. So, this is my DM. And this is Y cm. Substitute there an integrate 0 to capital R. Like that will happen now. Even in school exam, you cannot do that. Forget about it. You have to understand. If you just sit at home and keep on writing lab records, that is not studying. Studying is number of problems you should count. If you are not solving any problem, don't expect marks are not improving. The reason, surprisingly that you might So, pay attention. That is a direct correlation. Finding the mass of various problem that we asked in the JMAs. If you solve little bit more problems at home. Okay. Fine. So, this is the we have just started the chapter. And we have introduced how to find the location of the system.