The lever arm of the gravitational force on the 60-kg particle changes during the motion,
so the torque changes, so the angular acceleration changes. The projectile moves with
changing net acceleration and changing tangential acceleration. The ratio of the particles’
distances from the axis controls the ratio of their speeds, and this is different from
the ratio of their masses, so the total momentum changes during the motion. But the
mechanical energy stays constant, and that is how we solved the problem.