Question

(I)WHY NORMAL FORCE IN CASE OF HINGES ACTS IN OUT WARD DIRECTION?
EXPLAIN WITH THE HELP OF A DIAGRAM
(II)WHY DOES NORMAL REACTION BECOME ZERO EVEN THOUGH THE BODIES ARE IN CONTACT WITH EACH OTHER?
(III)EXPLAIN WEIGHTLESSNESS AND WHY DOES IT OCCUR IN CASE OF A LIFT AND WHAT HAPPENS THERE WHETHER LIFT GOES UP OR DOWN AND WHY PERSON LOSES CONTACT WITH LIFT AND EXPLAIN THE CONCEPT BEHIND THIS
(iv)WHY CANNOT WE POLISH SURFACES UPTO A POINT WHERE FRICTION BECOMES 0 OR NEGLIGIBLE?WHAT HAPPENS IF WE POLISH BEYOND THE LEVEL?WHAT IS THAT LEVEL?

Answer 1

1) Normal force is the reaction force from the hinges on to the door or other object in touch with it.  The force on the hinges can be split in to two  components .  One along the vertical direction or along the hinges.  One perpendicular to the hinges.

  The friction force acts along the surface.  The other component is called the normal force. It is the reaction force for any pressure acting on the hinges due to door or other object.

2)  In a freely falling lift, the bodies are in touch with each other on the floor of the lift.  But the object on the floor does not exert any weight on the floor and the floor does not exert any normal reaction on the object.  They are falling together at the same acceleration due to gravity...  This is the same situation in a satellite around a planet.

3)  Weight - Reaction = mass * acceleration
       Reaction force  = mg  - mg    as  a = g..

   the feeling of having a weight comes from the reaction force on the lift/floor on the persons' feet...  if there is no reaction from floor, the person feels weightless...

4)  if friction is zero, then the surfaces in contact with each other do not affect each other at all.  they slide on one another, but nothing happens to either surface...
   so if friction is zero, then no more polishing can be done...

   only if friction is present, then the dust from the shoe surface can be taken out of there..