Question

When a horse suddenly starts running the rider falls backward $\to$ inertia of rest

Answer 1

When a horse suddenly starts moving, the rider falls backward due to the law of inertia or 1st law of Newton.

Because the body was in the state of rest

and when the horse suddenly starts moving the lower body tends to be in motion

but the upper body still remains in a state of rest due to which it feels a jerk and falls backward.