of the disk and kinetic energy of the falling mass. Equating the initial and final states and using the relationship between v and ω the speed of M can be found. Mechanical energy is conserved even with the nonconservative tension force because the tension force is internal to the system (pulley, mass, rope).
Strategy 3: In trying to find the speed of the block I would try to find angular momentum kinetic energy, use gravity. I would also use rotational kinematics and moment of inertia around the center of mass for the disk.
Strategy 4: There will be a torque about the center of mass due to the weight of the block, M. The force pulling downward is mg. The moment of inertia about the axle is 1/2 MR2. The moment of inertia multiplied by the angular acceleration. By plugging these values into a kinematic expression, the angular speed can be calculated. Then, the angular speed times the radius gives you the velocity of the block.
The first two strategies display an excellent understanding of the principles, justification, and procedures that could be used to solve the problem (the what, why, and how for solving the problem). The last two strategies are largely a shopping list of physics terms or equations that were covered in the course, but the students are not able to articulate why or how they apply to the problem under consideration.
Having students write strategies (after modeling strategy writing for them and providing suitable scaffolding to ensure progress) provides an excellent formative assessment tool for monitoring whether or not students are making the appropriate links between problem contexts, and the principles and procedures that could be applied to solve them (see Leonard et al., 1996).