 the figure there is a rod all right so on the rod at the bottom there is a small heavy block let's say mass is m and length of the rod is l all right now this mass can be rotated about its clamped other end so about this end this mass can be rotated so this mass can be rotated like that okay so it goes on a vertical circle all right what is a minimum horizontal velocity should the block be given so that it moves in a complete vertical circle all right so it is asking what is this minimum velocity let's say you for which this block will complete the circle all right now you might have done a similar question when it was not a rod it was a string okay but in this case we are dealing with rod not the string okay so there is no worry of string becoming slack all right so it does not matter because a rod can create a compressive force and it can also create a tensile force so rod will support both types of forces fine so there is no constraint as such with respect to tension becoming zero now the only way it will complete a circle that it reaches the top of this circle isn't it because the topmost point is the critical point out here as it moves up the potential is increasing so its kinetic energy will go down so at the max at the max height the kinetic energy will be minimum right so if if I am talking about the limiting condition or this minimum velocity u I will ensure that entire kinetic energy gets converted into potential energy fine so now we have point number one and point number two so if I use work energy theorem which is w is equal to u2 plus k2 minus u1 plus k1 I see that here we don't have you know a force other than gravity which is doing any work right and for gravity you have already considered the potential energy right you can say that work done by the raw sorry the force due to the rod is there okay but that force is always towards the center whether it is compressive or tensile force it is always along the radius and the movement is tangential it is along the circle so the force is always perpendicular when I'm talking about the force due to rod right so work done in this case will be zero okay u2 is what the final potential energy of the block when it reaches the top that is equal to what mg into 2l okay and kinetic energy is what at point two kinetic energy is zero at point two the velocity goes to zero all right u1 is what initial potential energy I'm taking that as zero right I can take any horizontal line and I can assume that to be every zero potential energy so this is zero gravitation potential energy this is a reference fine and k1 is what k1 I can write as half m into u2 all right now when you solve this particular equation you will get u is equal to under root of 4g l okay so this much meter per second should be the velocity of the block so that it reaches that top most point and at the top most point if it has lightness of the movement this side it will again move that side and then it will be able to complete the circle okay so that's it with respect to this particular question so I hope you have learned something today in case you have any doubts please feel free to get in touch with us okay thank you