Let the whole system move upward with an acceleration a 1 relative to the bottom, and let the mass m2 move downward with an acceleration a2 relative to the pulley; The light rope and the light pulley are known, and the tension in the rope is equal everywhere. Suppose the tension of the rope on the two mass blocks is F 1, and the direction is upward. The ground is selected as the reference frame, the phase is selected in the positive direction, and the dynamic equation of the mass block is listed.

For m1:f1-m1g = (a1+a2) m1; Note: The left side of the equation is the resultant force of m 1, and the right side of the equation is the acceleration and mass product of m 1 relative to the ground.

For m2: f1-m2g = (a1-a2) m2; Note: the left side of the equation is still the resultant force, and the right side is the product of acceleration and mass relative to the surface. Note that the acceleration of the mass relative to the elevator should be negative here;

For the whole system: 300-(m1+m2) g = (m1+m2) a1; Explanation: the whole system on the left side of the equation is subjected to combined external force (regardless of internal force), and the product of system mass and acceleration on the right side.

Among the above three formulas, there are just three unknowns, namely, the rope tension F 1 and the acceleration a 1 and a2, and the solution is very simple. Answer: If the solution of a2 is less than zero, it means that the initial assumed acceleration direction of mass is opposite to the actual situation. Of course, this will only happen if the contrary assumption is made intentionally. It is entirely possible for the subject to assume that the mass block with large mass accelerates upward, and the acceleration of the solution will be less than 0, which means that the actual result is opposite to the assumed direction.