Fictitous Force and Free falling Elevator

A non inertial Frame of Refrence is the frame of refrence which is having acceleration(a₀) Example Free Falling elevator

Newtons law are valid in inertial frame of refrence only. We cannot apply them as it is in non inertial frame of refrence as it left out the acceleration of the object because of the frame of refrence.

Let me provide simple example to illustrate this.

You are moving in car which is accelerating. From your frame of refrence,a block on the road would be accelerating in opposite direction.

Now if we apply newton's law, we will find acceleration to be zero as no net force is acting. But the block has acceleration from car frame of refrence.

So to fix this issue,we will add one fictitous quantity in the Newton law equation like

F+F₀=Ma

Where F₀ is the fictitous force and it is in opposite direction of accleration frame

F₀=-Ma₀

Where a₀ is the acceleration of Non inertial frame of refrence

Now let me further illustrate this with example of free falling elevator

A free falling elevator is an non inertial frame of refrence.The acceleration of the elevator as defined from the observer on the ground will be

g=-gk

Where k is the unit vector along vertical axis in the upward direction

Let us take an unattached object of mass M in the elevator.

From the frame of refrence of free falling elevator,Two forces are acting on the mass

Fictitous Force due to non inertail frame of refrence

F_fic=Mgk It is opposite to the acceleration of the elevator

Gravitional Force on the Mass

F_G=-Mgk

So the total Force on the body from elevator frame of refrence

F_T=F_fic+F_G=0

Which mean body is unacclerated in the elevator frame of refrence. The body will appear to be suspended in the air ,if it has no initial relative velocity to the elevator

Few more important things about Fictitous force

Fictitous force are called pseudo force.
It simple represent the effect of the acceleration of the non inertial frame of refrence
They dont have any physical significance
Whenever we work the problem in non inertial frame of refrence,always make sure to include fictitous force in the newton law