Question

A wooden block of 100kg is about to be pushed on a floor of coefficient of friction is $0.4$. What is the magnitude of friction on the wooden block when it is just pushed?
(A) 392N
(B) 294N
(C) 196N
(D) 490N

Answer 1

Hint: The force of friction is directly proportional to the normal force. The normal force is equal to the weight of the wooden block.
Formula Used: The formulae used in the solution are given here.
Force of friction $f = \mu N$ where $\mu $ is the coefficient of friction and $N$ is the normal force.
Now we know that the normal force is equal to the weight of a body.

Complete Step by Step Solution: Friction is a force between two surfaces that are sliding, or trying to slide, across each other. Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
The maximum amount of friction force that a surface can apply upon an object can be easily calculated with the use of the given formula:
$Friction = \mu N$ where $\mu $ is the coefficient of friction between the block and the ground and $N$ is the normal force.
Now we know that the normal force is equal to the weight of a body. Mathematically, $N = mg$.
Thus, the force of friction can be expressed as, $f = \mu N = \mu mg$.
It has been given that a wooden block of 100kg is about to be pushed on a floor of coefficient of friction $0.4$.
Thus, mass $m = 100kg$ and coefficient of friction $\mu = 0.4$.
Let the acceleration due to gravity, represented by $g$ be $9.8m{s^{ - 2}}$. Thus, $g = 9.8m{s^{ - 2}}$.
Now, the force of friction represented by $f$, is given by
$f = \mu mg = 0.4 \times 100 \times 9.8$
$ \Rightarrow f = 392N$
The magnitude of friction on the wooden block when it is just pushed is 392N.

Hence the correct answer is Option A.

Note: The causes of friction are molecular adhesion, surface roughness, and deformations. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds. For years, scientists thought that friction was caused only by surface roughness, but recent studies have shown that it is actually a result of adhesive forces between the materials. But surface roughness is a factor when the materials are rough enough to cause serious abrasion. This is called the sandpaper effect. When one or both of the materials is relatively soft, much of the resistance to movement is caused by deformations of the objects or by a ploughing effect.