Elevator and inertia
Elevators are an excellent place to feel the inertia force.
Consider the case of taking an elevator and trying to go up from the lower floor.
When you press the button that goes up, you suddenly feel heavier. The elevator accelerates in the upward direction, but my body tries to stay by inertia. So the elevator floor puts pressure on my feet. If I stand on a scale, it will be measured as an increase in weight.
After the elevator has accelerated to some extent, it moves at a constant velocity. At this time, the elevator and the body move at a constant speed. The relative speed between the elevator and the body is '0'. If I'm standing on a scale, my weight is measured the same as just standing on the ground.
When the elevator is almost arriving up, it slows down gradually. My body feels a little lighter because it keeps trying to rise by inertia. If I stand on a scale, it will be measured as my weight loss.
The force that changed my weight while the elevator was accelerating or decelerating is called the 'inertial force.' The 'force of inertia' is measured in proportion to the mass because an object's inertia expresses it. The interesting thing is that inertia is not a force that exists.
In this case, the only forces that exist are the 'electromagnetic force' of the elevator motor and the earth's gravity.
The force of inertia is not a force that actually exists.
You may have experienced your body sway outward when a bus turns sharply.
It's not because the bus pulls your body out. It's because my body is trying to an inertial motion.
As such, the force of inertia is not a force that exists.