Is the Second Hint for Newton's Law of Restitution Correct?

[Harmony]

Homework Statement

http://mediocre.wordpress.com/2008/01/09/Newtons-law-of-restitution/
Shown in my blog, link provided.

Homework Equations

Newton's Law of Restitution, v2 - v1 = e (u1 - u2)

The Attempt at a Solution

I have done the question, actually...but I need confirmation. Can anyone please help me to check whether the no.2 hint is correct or not? (Located at the bottom of the post)

Thanks in advanced.

 

[Shooting Star]

Quite correct. The coefficient of restitution is the ratio of the magnitude of the relative velocity of separation to that of the relative velocity of approach in the normal direction.

 

[Harmony]

Yeah, but though I get the answer, I am not sure why only the vertical component of velocity is considered in the 2nd question. Or am I wrong?

 

[Shooting Star]

No, you are right. The vertical component is in the normal direction. For now, you can take it to be a law of nature.