Why the rocket moves slower than the planet?

[JERONIMO]

It's more of a physics question probably but why does the rocket after launch starts moving (or lagging behind) kerbin? Shouldn't it retain it's motion after launch?  Why does it go in the opposite direction of the planets rotation?

[Superfluous J]

I'd need to see a picture to know exactly what you're talking about, but it's likely that Kerbin is rotating under you and your ship was sent straight up. It's just an illusion that you're going "backwards". In fact you're standing still (floating still?) in space and Kerbin is rotating under you.

Even if you retain the 250 or whatever m/s of Kerbin's surface rotation, it is moving 250m/s around its relatively smaller circle, while the 250m/s you retained is around a much larger circle up in space. When it's rotated halfway around, you've gone 10% of your orbit.

[king of nowhere]

10 hours ago, JERONIMO said:

It's more of a physics question probably but why does the rocket after launch starts moving (or lagging behind) kerbin? Shouldn't it retain it's motion after launch?  Why does it go in the opposite direction of the planets rotation?

You mean if you go straight up?

it's called the coriolis effect. @Superfluous J gave a quick description, but you can search the term for more information