masses. (The rotational inertia for a solid disc of mass m and radius r is ½mr 2 .)We learned how to approach this type of problem in Chapter 12 —draw a free-body diagram for each
block, and use F (^) net = ma . So we start that way.
The twist in this problem is the massive pulley, which causes two changes in the problem-solving
approach:
We have to draw a free-body diagram for the pulley as well as the blocks. Even though it doesn’t
move, it still requires torque to accelerate its spinning speed.
- We oversimplified things in Chapter 12 when we said, “One rope = one tension.” The Physics C
corollary to this rule says, “... unless the rope is interrupted by a mass.” Because the pulley is
massive, the tension in the rope may be different on each side of the pulley.
The Physics C corollary means that the free-body diagrams indicate T 1 and T 2 , which must be treated as
different variables. The free-body diagram for the pulley includes only these two tensions: