CBSE Class 12-science Answered

In Newtonian mechanics, it is assumed that measurements and observations are made in inertial frame of reference.

( Inertial frame of reference means frame of reference either at rest or moving with constant speed ) . By Newton’s first law,

the object will not be accelerated If there is no net force acting on the object. Though we prefer most of the time to choose

inertial reference frame , some time it is convenient to choose non-inertial frame to analyze the dynamics of objects .

Non-inertial frame of reference is a frame attached to the object that is in accelerated motion as viewed by observer from

inertial frame of reference. Frame of reference attached to accelerating car and frame of reference attached to merry-go-round

are some of the examples of non-inertial frame.

To apply the principle of mechanics in non-inertial frame, pseudo force is introduced. Pseudo force  is simply a device that permits

us to apply the principle of mechanics in normal way to the events if we insist on the viewing events from a non-inertial frame of reference.

We cannot associate the pseudo force with any particular object in the environment of the object on which it act.

We cannot classify the pseudo force into any of the category as contact force , friction force etc.. Pseudo force acting on the object

in non-inertial frame  does not appear to the observer observing the object from inertial frame .

Pseudo force is understood from the following example. Let us consider an observer S’ is  in a car that is moving with  constant velocity .

Let a small spherical ball is suspended from the ceiling of the car.  Since the car is moving with constant velocity ,

it appears to the observer S’ that the ball is hanging vertically. Let the driver applies break to slow down .

Let an observer S on the road who is in inertial frame of reference observes the car slows down with acceleration a = -2 m/s² .

Hence the observer S’ is in non-inertial frame. Observer S’ will observe the sphere moving forward with acceleration a = 2 m/s² .

Let us see how each observer is accounting the motion of spherical ball using newton’s second law.

To the ground observer S who is in inertial frame of reference , the ball is moving forward at constant velocity before brake is applied.

Hence for him, the ball continues to moving forward . According to observer S, the ball has no acceleration hence no force is acting on it.

Observer S’ will see the ball is moving forward in accelerated motion.  Observer S’ can not find any object exerts force on the ball

to move in accelerated motion. Hence , to apply Newton’s second law , observer S’ must assume a force, in this case pseudo force,

acts on the ball. This pseudo force equals F = ma’ , where m is mass of ball and a’ = - a is the acceleration measured by observer S’ .

This pseudo force which is real from the point of view of observe S’ , is not apparent to the ground observer S who is in inertial frame.

Pseudoforces are very real to those experience them. Imagine someone in car moving in a curve to the left. 

Let us consider a ground observer viewing the person in the car in straight line .To the ground observer  the car experiences 

centripetal acceleration and therfore person in car is in a non-inertial frame. If friction is negligible in the seat,  person in the car will find

that he is sliding towards right . To a ground observer , this event is seen as if the peron in the car moving in straight line

which is his natural motion but the car is moving left . From the person in car point of view , since he is in non-inertial frame ,

he will think that his sliding motion is due tp pseudo force that is pushing him towards right. This type of pseudo force is called a

centrifugal force that means force directed away from centre.