～くする or ～にする versus ～くなる or ～になる

Question

According my teacher,

• ～くする or ～にする is used for direct action, but
• ～くなる or ～になる is used for indirect action

For me, the meaning of direct and indirect is a bit ambiguous. Consider there is a tool than can cut something. The tool can be either powered by a motor or powered by human energy. Is the decision whether directly or indirectly based on the source of energy to do the action?

I mean the sentence A below will be true if the tool is powered by a motor, for example. Otherwise it will be false if powered by a human.

A: このツールを使うと、ズボンが短くなります。

With almost the same logic, the sentence B below will be true if the tool is powered by a human. Otherwise it will be false if powered by a motor.

B: このツールを使うと、ズボンを短くします。

What do you think? More precisely, when do we have to use する and なる? Any comment is appreciated.

Answer 1

A: このツールを使うと、ズボンが短くなります。

If you use ～くなる or ～になる here, you intend to mean this: "If you use this tool, the pants becomes smaller."

B: このツールを使うと、ズボンを短くします。

If you use ～くする or ～にする here, you intend to mean this: "If you use this tool, you make the pants smaller."

As opposed to ～くなる or ～になる, ～くする or ～にする implies that you have made a decision or have made an action yourself.

～くなる or ～になる on the other hand, you're just making a statement or an observation that something has undergone change.

Hope this helps.