How Does Force Affect the Acceleration of Block B in a Pulley System?

[Prof. 27]

Homework Statement

The masses of blocks A and B in the figure (Figure 1) are 20.0 kg and 10.0 kg, respectively. The blocks are initially at rest on the floor and are connected by a massless string passing over a massless and frictionless pulley. An upward force F⃗ is applied to the pulley.

Find the acceleration b of block B when F is 294 N?

Homework Equations

F = ma

The Attempt at a Solution

Hyperphysics and Physics Clasroom Reading
Looking at past problems I got right that were similar.

Essentially, I get it right that the acceleration a of block A is equal to zero at that force and that at 124 N both blocks have acceleration 0, but doesn't that mean that the acceleration of block B is always zero since the it didn't accelerate at 124 N? That was a net force, so it should have caused acceleration yes?

 

[haruspex]

Homework Statement

The masses of blocks A and B in the figure (Figure 1) are 20.0 kg and 10.0 kg, respectively. The blocks are initially at rest on the floor and are connected by a massless string passing over a massless and frictionless pulley. An upward force F⃗ is applied to the pulley.

Find the acceleration b of block B when F is 294 N?

Homework Equations

F = ma

The Attempt at a Solution

Hyperphysics and Physics Clasroom Reading
Looking at past problems I got right that were similar.

Essentially, I get it right that the acceleration a of block A is equal to zero at that force and that at 124 N both blocks have acceleration 0, but doesn't that mean that the acceleration of block B is always zero since the it didn't accelerate at 124 N? That was a net force, so it should have caused acceleration yes?

It was the net force on what? That force acts on the pulley, but other forces act on the pulley.
Where did the 124N come from, it's not in the problem statement as given.