I know that momentum is conserved if there are no external forces. But, can you tell me what external forces are ? The site http://www.physicsclassroom.com/class/energy/Lesson-2/Internal-vs-External-Forces says that internal forces are force of gravity and force of spring. But, I think that force of gravity causes acceleration that makes the momentum increases, so it is not conserved. Dropping a ball from some height causes the ball accelerate and momentum changes. So, what are external forces and internal forces ? Are force of spring and gravity external forces ?