Can velocity change without any component of acceleration in opposite direction of motion? If a truck of mass 100kg  is moving  with some velocity say 10m/s ( pls assume  friction is negligible  but still truck is moving)when suddenly  an object of mass 10kg falls on it  from above and stickes to it ,such that the object has 1m/s speed (say) in the downward direction  when it was about touch the truck. Now the velocity  of the system  decreases a/c law of conservation  of momentum. But if change in velocity has to occur, there must be an acceleration  in the opposite direction of the motion of the truck,  but since this is an isolated system no external  force is possible.  Only  forces acting are internal forces btw object  and truck  in the perpendicular  direction to the motion of truck. How did the velocity change without  any component of acceleration in the opposite direction of the truck?
Pls clear my confusion!!!!!
Thank you
 A: After the $10$ kg mass falls onto the truck here must some force acting on it to prevent it sliding straight off the back of the truck. This could be friction, or it could hit a wall at the back of the truck. In any case, the truck is exerting some force on the $10$ kg mass to increase its horizontal sped from $0$ to $10$ m/s. By Newton's Third Law the $10$ kg mass exerts an equal and opposite force on the truck which slows the truck down (assuming there is no driving force maintaining the truck's speed).
A: 
Can velocity change without any component of acceleration in opposite direction of motion?

Simple answer is NO, it can't. Falling object induced a change in rolling friction force :
$$
\begin{align}
 \Delta F&=C_{rr}\Delta N 
\\&=C_{rr}~m_o\left(\frac{v_i}{\Delta t}+g\right) 
\end{align}
$$
Where $m_o$ is falling object mass, $v_i$ is object impact speed, $\Delta t$ impact duration. So object increases rolling resistance and as such - produces negative acceleration, due to friction change.
EDIT
Complex answer is, in some cases it can. For example if an object is rotating in a circular motion due to some centripetal force, then lowering centripetal acceleration for an object at a fixed radius, it's tangential speed perpendicular to centripetal acceleration vector will drop too. But your example is about movement in a straight trajectory, so this is not the case here.
