WHY WE SAY THAT IN ANY EXAMPLES OF INERTIA
When a horse starts suddenly, the rider falls backwards due to the inertia of rest of the upper part of his body. why only his upper body is in inertia?
The lower part of the body (including waist and legs) is joined with the cart itself while the upper part is free. This is why only the upper part of the body is project backwards or forwards. The center point of the body acts as a pivot around which the upper body turns.