Frictional force is defined by the following formula -
f = coefficient of friction (u) X Normal reaction exerted by the surface (N)
So, f = u X N.
Now 'u' is a constant for given surfaces in contact ( keep in mind that it depends on both the surfaces that are in contact with each other, here wall and the book that you are pressing)
Now, Normal reaction is equal and opposite to the force exerted by an external force on the surface, PERPENDICUAL TO THE SURFACE ( or the perpendicular component of the external force). The external force exerted on the surface which is vertical to it, in this case, is equal to the force that you are applying while you are holding/pressing the book against the wall.
So, at any point of time, frictional force = u X (Force you are applying on the surface, via the book)
Or, f = u X F, where,
f = frictional force,
u = coefficient of friction, and
F = Perpendicular component of the force exerted by you on the book towards the surface.
Now direction of f -
Frictional force always act parallel to the surface and in opposite direction to the direction of motion.
In absence of friction, book will fall down, because of gravity acting on it. So friction will try to act in the opposite direction of this motion. So, upwards in this case.
If let's say you have a metal object placed between magnets, which is applying more force on it than gravity, in upwards direction; and you try to press it against the wall, the motion of object will be upwards, so friction will then act downwards.
Friction always opposes the direction of motion in which the object would have moved because of RESULTANT FORCE on it in absence of friction.
Takeaways in English (or, common observation) -
1) Harder you press the book against the wall, harder it will be for a person to move the book on the surface of the wall, while you are pressing it. Because of higher friction.
2) The more you move the book on the wall, while keep on pressing it, warmer the surface of the book which is in contact with wall will be. Because if more energy is dissipated per second to overcome friction, which gets converted to heat energy.
2.1) Harder you press, book will heat up faster.
3) Harder you press the book against the wall, lesser chance are that it will fall down. Because more frictional force is now acting on it to hold it against gravity.
4) Heavier the book, more frictional force is needed to hold it from falling, and that is why you have to press it harder to hold Heavier books.
5) That is all I can think of right now. Try applying this concept in things you see around.
12.8K viewsView 6 Upvoters · Answer requested by Muhammad Shouman
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