Suppose we have two small masses both with the same velocity. Their mass is $m$ and velocity is $v$ and $-v$ respectively. So they collide at some point and the collision is a perfect elastic one. We mark this point as the origin.
Now we can know the velocity of any particle which will be $v$. So its momentum will be $mv$. Now after $t$ seconds its displacement from the origin will be $vt$.
Now we know the momentum and position of the small mass at the same time. Isn't it the violation of the uncertainty principle? Where am I wrong?