When a fast-moving horse stops suddenly,a careless rider falls in the forward direction. Give the reason for this.

(N/A) This happens due to the $inertia$ of motion.
When the horse is in motion,the entire body of the rider is also in motion.
When the horse stops suddenly,the lower part of the rider's body,which is in contact with the horse,comes to rest due to the horse's deceleration.
However,the upper part of the rider's body continues to remain in a state of motion due to the $inertia$ of motion,causing the rider to fall forward.