 Alright guys, so here is a question that is asked by one of our students, so let's see what is this question about. So you have a disk, so this disk has a radius of 2R and what you are doing is you are putting another disk on top of it which has a radius R. So this smaller disk is located like this as shown in the figure, it touches the periphery of the bigger disk. So let's say this is radius 2R and this disk which is on top of it, let me put this in a different color, so this is another disk, so this particular disk has radius R. So what we need to find out is the center of mass. Now we know that center of mass will lie on the axis of symmetry, so there is a line of symmetry over here, so this is the line of symmetry, so center of mass will be somewhere here, but then there is only one axis of symmetry, so we will not be able to pinpoint where exactly in this line the center of mass is. So what I will do, I will represent another line perpendicular to it and show that this line as y axis and this line I will show as x axis. So basically the y coordinate of center of mass is 0 because x axis is the line of symmetry. Now we need to find out the x coordinate of center of mass. So simply what I will do, I will use this formula m1 x center of mass of the first mass plus m2 x center of mass of the second mass divided by some of the two masses m1 and m2. Now here you can see that individually finding the center of masses are equal, are easy. So for example, x center of mass for the first mass is what, m1 is let us say the bigger ring, so center of mass for the bigger ring is the original ring, so x center of mass 1 is 0 and center of mass for the second mass which is the smaller ring is what, is the center of the smaller ring whose coordinate is r comma 0, isn't it? Now let us try to write down the masses also, there should be a relation between the masses as well between the smaller and the bigger disc because they are made up of same material and their thicknesses are same. So let us say that surface mass density is sigma. So sigma into pi r square is the mass of the second ring, the smaller ring. So I am multiplying the area of the smaller ring with mass per unit area, so I get m2. Similarly if I multiply sigma with the bigger area which is this, I will get m1. So basically the relation you will get here is that m2 is 4 times of m1. Now let us use all of this and substitute in this particular formula. So what do I get here is that x center of mass is m1 which let us say I am sorry here m1 is equal to 4 times of m2, so make a correction. So x center of mass is m1 which is actually 4 times of m2, so I will write mass in terms of m2 only now into 0 plus m2 multiplied by its center of mass which is r divided by total mass which is what 4m2 plus m2. So now you can see that this anyway goes to 0 and m2 get cancelled off and what you get here is r by 5. So the x corner of center of mass for this system is r by 5 from the center of the bigger disk. So like this you can solve this particular question, thank you.