Question

why static friction is more than kinetic friction?

Answer 1

First of all to understand why coefficient of static friction is greater than coefficient of kinetic friction you need to understand origin of friction.
There are multiple theories which explain the origin of friction and all of them give satisfactory explanation for this phenomenon.

One such theory is the "Adhesion Theory of Friction"

According to this theory, the surfaces in contact, however smooth they may appear, actually have imperfections called Asperities.

When one surface rests on the other the actual area of contact is very less than the surface area of the face of contact.

The pressure due to the reaction force between the surfaces is very high as the true contact area is very small.

Hence, these contact points deform a little and cold welds are formed at these points.

So, in order to start the relative sliding between these surfaces, enough force has to be applied to break these welds.

But, once the welds break and the surfaces start sliding over each other, the further formation of these welds is relatively slow and weak and hence a smaller force is enough to keep the block moving with uniform velocity.

This is the reason why coefficient of static friction is greater than coefficient of kinetic friction.

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Answer 2

Static friction is the friction that keeps an object in place.

This friction must be overcome by energy to move the object. Because of a combination of surface irregularities and electrical charges between the object and the surface, the energy required to initially move the static object is greater than the energy required to keep it in motion.

Because it takes more energy to move the static object, physicists say that static friction is greater than kinetic friction.

The smoother the object and the smoother the surface the smaller the difference between the amount of static friction and the amount of kinetic friction.

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