What kind of lens (converging or diverging) should he use to correct his eyesight, and what should be the focal length of the lens?

Incomplete question. However, I provided some information about when a converging or diverging lens is used. The common ones are:

Explanation:

In other to correct eyesight problems certain types of lenses are used depending on the condition.

Diverging lens: If the individual is suffering from nearsightedness (myopia); a condition that prevents a person from seeing distant objects clearly, but is able to see close objects clearly, then a divergent lens may be used to correct his eyesight.

Covergent lens: In a case where the person has a condition that prevents him from seeing close objects clearly while distant objects may be clear is called Farsightedness (hyperopia).

the bal will not change its velocity.

talk back to him about the sport and show him your expertise or leave the room