# Rotational mechanics

1. May 15, 2012

### joker94

Two solid cylinders p and Q of same mass and same radius start rolling down a fixed inclinded plane from the same height at the asme time.cylinder P has most of its mass concentrated near its surface while Q HAS MOST of its mass near the axis....
THEN
1)WHICH HAS MORE MOMENT OF INERTIA AND WHY???
2)WHAT IS THE INSTANTEOUS AXIS OF ROTATION??
3)DOES BOTH CYLINDERS REACH THE GROUND WITH SAME TRANSLATIONAL KINECTIC ENERGY???

2. May 15, 2012

### tiny-tim

welcome to pf!

hi joker94! welcome to pf!

tell us what you think, and then we'll comment!

3. May 15, 2012

### joker94

hi......can we consider P as a hollow cylinder then moment of inertia about the axis(which is passing through its centre) would be MR^2....AND THEN WE CAN CONSIDER Q AS A SOLID CYLINDER HAVING MOMENT OF INERTIA MR^2/2 (^2 MEANS SQUARE AND / DIVIDE)......SO IT MEANS MOMENT OF INERTIA OF P IS MORE THAN MOMENT OF INERTIA OF Q.!!!
AM I CORRECT???

4. May 15, 2012

### haruspex

Right answer, though of course it would be better not to have to assume the extreme case when the original question was posed more generally.