If the horse starts moving suddenly, the possibility of the rider falling is due to inertia. Inertia is the property of matter that resists changes in motion. When the horse accelerates forward, the rider tends to remain at rest due to their inertia. This causes the rider to lean backward or lose balance, potentially leading to a fall. This phenomenon aligns with Newton’s first law of motion, often referred to as the law of inertia. According to this law, an object at rest will remain at rest unless acted upon by an external force. In this scenario, the sudden motion of the horse acts as the external force, causing the rider to experience a change in motion. Therefore, the possibility of the rider falling when the horse starts moving suddenly can be attributed to the principle of inertia described by Newton’s first law of motion.