Newton law

[neik]

Does anyone know how to solve this?
http://img.photobucket.com/albums/v64/neik7/111.jpg

[Pyrrhus]

Many here know how to, but we are not going to solve it, you will with our help.

First the problem states the particles moves continously with that velocity, so do you know Newton's 1st Law?

\sum_{i=1}^{n} \vec{F}_{i} = 0 \rightarrow \vec{v} = constant