Question

A box is lying on a rough floor. What is the maximum value of Fext so that box do not slip relative to the floor?​

Answer 1

Explanation:

The floor applies 2 forces on the box. Normal reaction force and frictional force. 

The direction of the normal reaction force is upwards. (this is to counter the gravitational force which acts downwards)

Since the train decelerates, it means its acceleration is towards the left (the direction of acceleration is opposite to the direction of motion)

Friction acts in the direction of acceleration so as to prevent relative motion between the floor of the train and the box.

Therefore friction acts towards the left and normal reaction force acts upwards. The resultant force will be as in option C