1. The problem statement, all variables and given/known data If two circular discs A and B of mass m and 3m and radii R,2R, respectively. are placed from the top of a rough inclined plane, which disc will reach bottom first. 2. The attempt at a solution NOTE: u is coefficient of friction (both kinetic and static). g is acceleration due to gravity & t is inclination of inclined plane. friction on disc of radii R is u*g*cos(t) and that in 2R is 3g*u*cos(t). In starting there will not pure rolling but after some time there will pure rolling(may not if inclined is too small). In any case friction force will remain as it in starting(u*g*cos(t) and u*g*cos(t)). so linear acceleration of both the objects will different so their time to reach bottom should different(Heavier should reach late on bottom larger retardation). BUT MY EXPLANATION IS WRONG WHERE SM I WRONG. that's my question to you. please try to answer with good answer. Translational and rotational motion are confusing my mind. Do you have any way to understand rotational in better way.