Question

A block of mass $4 \mathrm{kg}$ is kept on a rough horizontal surface. The coefficient of static friction is 0.8. If a force of $19 \mathrm{N}$ is applied on the block parallel to the floor, then the force of friction between the block and floor is:
(A) 32N
(B) 18N
(C) 19N
(D) 9.8N

Answer 1

Hint: We know that a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), that is to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F. Force is the push or pull applied on an object. It can move a stationary object or stop a moving object. Force can also change the speed and direction of a moving object. When force is applied on an object, resulting in the movement of that object, work is said to be done. Based on this concept we have to solve this question.

Complete step by step answer
We know that,
Static friction is a self-adjusting force.
Therefore, the limiting value of friction up to which the body stays at rest till it starts moving is given by, ${{F}_{\text{s}}}=\mu \text{ mg}$
where $\mu$ is the coefficient of static friction. Below that limit, the frictional force is equal to the force applied. Here $\mathrm{F}_{\mathrm{s}}=0.8 \times 4 \times 10=32 \mathrm{N} ; \quad \mathrm{g}=10 \mathrm{m} / \mathrm{s}^{2}$
Whereas force applied is $19 \mathrm{N}$ less than the limiting value of friction. Hence Frictional force is $19 \mathrm{N}$.

Therefore the correct answer is Option C.

Note We know that static friction is a force that keeps an object at rest. Static friction definition can be written as the friction experienced when individuals try to move a stationary object on a surface, without actually triggering any relative motion between the body and the surface which it is on. It hinders the movement of an object moving along the path. When two fabrics slide over each other, this friction occurs. There's friction all around us. When we walk, for instance, our feet are in touch with the floor. Static friction is caused by adhesion, light chemical attraction between two surfaces. And friction, in general, is caused by the imperfections in every surface gripping together and overlapping.
It should also be known to us that kinetic friction (also referred to as dynamic friction) is the force that resists the relative movement of the surfaces once they're in motion.