Question

When Mahesh Babu increases his running speed, he experiences ${f_1}$ friction and when he wants to decrease his speed at a fast rate, he experiences ${f_2}$ friction.
A. ${f_1}$ is static and ${f_2}$ is kinetic
B. ${f_1}$, ${f_2}$ both kinetic
C. ${f_1}$, ${f_2}$ both static
D. ${f_1}$ is kinetic and ${f_2}$ is static

Answer 1

The concepts behind static and kinetic types of friction must be applied to determine whether the friction caused is static or kinetic. As the name suggests the static friction applies for a body at rest while kinetic friction is considered when the body is already in motion.

Complete step by step answer:
When a person is running a frictional force is experienced by him as friction comes into picture whenever two bodies come in contact with each other and this frictional force opposes the relative motion of the body. Here, the person running is the body who is in contact with the ground surface which applies the frictional force. The two types of friction taken into consideration here are the static and the kinetic frictions.

Static frictional forces are forces that oppose impending motion which means that there is no relative motion but it is on the verge of moving or slipping. A body undergoes static friction only when there is an external applied force on the body when the body is at rest.When the applied force exceeds the maximum limit of static friction the body begins to move. By making use of this concept and from the definition of static friction we can see that ${f_1}$ friction is not static friction as the body is already in motion and the person is trying to increase his speed.

Kinetic friction comes into picture once the body is in motion and this type of friction opposes the relative motion of the body along the ground surface while running. Also, it is seen that there is no external force that is being applied to the running man; he is not being pushed and hence when there is no external applied force there is no static friction.Hence ${f_1}$ friction is kinetic.

The body in motion is trying to decrease its speed and hence here also the concept of kinetic friction is applied and the person is already in motion and is trying to slow down. This uses the opposition of the kinetic friction of the ground surface.Hence ${f_2}$ friction is kinetic.This means that ${f_1} = {f_2} = {f_k}$ where the subscript $k$ denotes kinetic friction. Since both ${f_1}$ and ${f_2}$ are kinetic frictions, option B is the right option.

Hence, the correct answer is option B.

Additional information: Kinetic friction is usually lesser than maximum value of static friction. The value of kinetic friction and static friction are independent of the area of contact and the velocity (if the velocity is not too high). The kinetic and static frictional forces are directly proportional to the normal reaction force which is the default force that exists between the two surfaces in contact. Hence, the equation is given by:
${f_k} = {\mu _k}N$ for kinetic friction
${f_s} \leqslant {\mu _s}N$ for static friction
where, $N$ is the normal reaction force, ${\mu _k}$ is the coefficient of kinetic friction (proportionality constant) and ${\mu _s}$ is the coefficient of static friction.
This coefficient of kinetic friction is also lesser than the coefficient of static friction.

Note: Static friction is a self-adjusting type of friction. It is said to restrict impending motion but in reality the motion does not take place because this type of friction comes into picture before one body actually starts moving over the other. However, kinetic friction is the friction which occurs when there is a steady motion between the two bodies in contact.