Final answer:
The opposite of 'powerful' with a changed suffix is 'powerless', which is derived by using the suffix '-less' indicating the lack of something.
Step-by-step explanation:
The opposite of the word 'powerful' when the suffix is changed is powerless. The suffix '-less' indicates the absence of something, therefore 'powerless' means without power, which is directly opposite to 'powerful' meaning having power. If we look at the provided suffix list, specifically at 6.14: Four Suffixes--ful, -less, -ly, and -y, it supports the idea that '-less' is the correct choice to form an antonym. The word 'powerful' conjures many images of strength and energy as indicated by the examples provided: a professional football player, a dragster, a volcano, or a rocket.