Question

For ordinary terrestrial experiments, which of the observers below are inertial and which are non-inertial?
(A) A child revolving in a giant wheel.
(B) A driver in a sports car moving with a constant high speed of 200 km/hr on a straight road.
(C) The pilot of an airplane which is taking off.
(D) A cyclist negotiating a sharp turn.
(E) The guard of a train which is slowing down to stop at a station.

Answer 1

Hint To answer this question, we need to evaluate all the options to see if they are at rest or moving at constant velocity or moving under acceleration. Only those options in which the observer is at rest or moving with a constant velocity are inertial and the others are non-inertial.

Complete step by step answer
An inertial system is said to be a system that follows the Law of inertia according to which a body at rest or in uniform motion continues to stay at rest or in uniform motion until an external force is applied on it. Thus, deriving this concept, an inertial frame of reference is where the observer is at rest or in uniform motion. The important point here is that an observer who is accelerating is not inertial as his/her speed is changing continuously.
In the first option, the child is in an accelerating frame of reference as his direction of motion is continuously changing which is causing a centripetal acceleration. Thus he is non inertial.
In the second Option, since the driver is moving with a constant uniform speed on a straight road, neither his velocity nor his direction is changing and hence he is inertial.
In the third Option, since the plane is taking off, the pilot is subjected to acceleration and thus the pilot is non-inertial.
In the fourth option, since the cyclist is negotiating a sharp turn, he is subjected to centripetal acceleration and so he is non-inertial.
In the fifth option, since the train is slowing down, which means that it is decelerating and its velocity is changing, therefore, the guard on the train is non-inertial.
$\therefore$ Option (B) is only inertial and the rest of the options are non-inertial.

Note
Do not consider the frames moving in a uniform circular motion as the inertial frames. Although they have a uniform speed, the direction of velocity keeps changing continuously due to which the frame becomes non-inertial.