Question

A rider on horse falls when horse starts running all of a sudden because

Answer 1

According to Newton's first law of motion an object in motion remains in motion and an object at rest remains in rest(until an external force is applied to it) . This is also called the law of inertia. So the person on the horse is in rest, and this tries to remain at rest. But the part of the rider in contact with the horse moves with the horse. So when the horse suddenly moves... The upper body of the rider tries to stay at rest, but his lower portion moves with the house. Thus the rider falls.