It happens due to inertia of rest. Before the horse starts moving, the person is at rest. When the horse moves suddenly, the person’s body tends to remain at its previous position which is behind the horse. As a result the person falls off from the horse.

Example A rider sitting on a horse back falls backward, if the horse gallops off suddenly. It is because when the horse suddenly sets itself in motion, the rider on account of inertia of rest, tends to continue in the state of rest. Thus, the rider is left behind the horse and hence falls.

A rider on horse back falls when horse starts running all of a sudden because. of inertia of motion. inertia of rest keeps the upper part of body at rest whereas lower part of the body moves forward with the horse.

As body tends to be in its original state when the horse suddenly stops, the lower part of rider stops with it but upper part remains in motion. So, rider falls forward.

When the horse suddenly stops, the rider falls in the forward direction due to the inertia of motion. The lower portion of the rider comes to rest along with the horse while the upper portion of the rider still continues to move forward. Hence, he falls forward.

When a horse suddenly starts moving, the rider falls backward due to the law of inertia or 1st law of Newton. but the upper body still remains in a state of rest due to which it feels a jerk and falls backward.

Explanation: A rider sitting on a horse back falls backward, if the horse gallops off suddenly. It is because when the horse suddenly sets itself in motion, the rider on account of inertia of rest, tends to continue in the state of rest.

When a horse suddenly starts running the rider falls backward → inertia of rest.

A rider on horse falls back when horse starts running, all of a sudden because the rider is in state of rest initially and later inertia of rest keeps the upper part of body at rest while lower part of the body moves forward with horse.

Inertia: tendency of an object to resist changes in its velocity. An object at rest has zero velocity – and (in the absence of an unbalanced force) will remain with a zero velocity. Such an object will not change its state of motion (i.e., velocity) unless acted upon by an unbalanced force.

Newton’s Second Law of Motion says that acceleration (gaining speed) happens when a force acts on a mass (object). Riding your bicycle is a good example of this law of motion at work. Your bicycle is the mass. Your leg muscles pushing pushing on the pedals of your bicycle is the force.

Newton’s Second Law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. ie., F=ma. Where F is the force applied, m is the mass of the body, and a, the acceleration produced.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, but as a consequence of masses moving along geodesic lines in a curved spacetime caused by the uneven distribution of mass.