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