Question 73: Laws of Motion

Question

Given below are two statements :
Statement (I) : The limiting force of static friction depends on the area of contact and independent of materials.
Statement (II) : The limiting force of kinetic friction is independent of the area of contact and depends on materials.
In the light of the above statements, choose the most appropriate answer from the options given below :

Accepted Answer

Let's analyze both statements:
Statement (I): The limiting force of static friction depends on the area of contact and independent of materials.
This statement is incorrect. The limiting force of static friction does not depend on the area of contact but is dependent on the materials in contact. According to the law of static friction, the maximum static frictional force $$ f_s $$ that can occur before motion commences is given by the product of the coefficient of static friction $$ \mu_s $$ and the normal reaction force $$ N $$:
$$ f_s = \mu_s 	imes N $$
The coefficient $$ \mu_s $$ is a property that depends on the materials in contact, not on the area of contact. The normal force $$ N $$ is the force perpendicular to the surfaces in contact, influenced by the weight of the object and any other perpendicular forces acting upon it.
Statement (II): The limiting force of kinetic friction is independent of the area of contact and depends on materials.
This statement is correct. Once an object is in motion, the kinetic frictional force $$ f_k $$ opposing its motion is given by the product of the coefficient of kinetic friction $$ \mu_k $$ and the normal force $$ N $$:
$$ f_k = \mu_k 	imes N $$
The coefficient $$ \mu_k $$, like $$ \mu_s $$, is also a property dependent on the materials of the surfaces in contact. It generally has a lower value than $$ \mu_s $$, which is why objects tend to be easier to keep moving once they've started. The kinetic frictional force is independent of the area of contact between the two surfaces.
Given these explanations, the correct answer would be:
Option A
: Statement I is incorrect but Statement II is correct.

Answer

Statement I is incorrect but Statement II is correct

Answer

Both Statement I and Statement II are correct

Answer

Both Statement I and Statement II are incorrect

Answer

Statement I is correct but Statement II is incorrect

Laws of Motion

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