text: why do things start to move, keep moving, slow down or stop? has to do with newton's 3 lasw of motion, every body has an inherent tendency to continue in its erstwhile state of rest or uniform motion when no external force is acting on it. therefore, bodies start to move, speed up, slow down or stop only when an external force acts on them and forces them to do so. for example, a ball rolling on the ground slows down and then stops because the force of friction acts in a direction opposite to the direction of motion and therefore decelerates it. this is newton's first law of motion.