Question

How average frictional force is calculated?

Answer 1

Hint: The frictional force typically works in the opposite direction of motion. It tends to oppose the motion and degrades the force applied to induce the motion as a large amount of force is utilized for overcoming the friction and only the remaining force can cause motion.

Complete answer:
The opposition experienced by an object to initiate motion or while in motion relative to a surface is known as friction. Unlike gravity or electromagnetism, the frictional force is not fundamental. It acts opposite to the direction in which the object or body is trying to move or moving.
There are two types of friction:
Static Friction \[\left( {{\mu _s}} \right)\]: The force of friction that causes the bodies at rest to be at rest is known as static friction. More simply, due to static friction, the bodies remain at rest.
Kinetic Friction \[\left( {{\mu _k}} \right)\]: The resistance to the motion of a body over a surface is known as kinetic friction. We know that once the body overcomes static friction then it starts moving, so the force of friction that acts between moving surfaces is the kinetic friction.
Calculation of average friction:
For static frictional force, it increases linearly w.r.t applied force, so by taking the mean of the maximum value of friction with the minimum value you can get the average force of friction.
Suppose, a mass of m was lying on a horizontal surface whose frictional coefficient is $\mu $.
So, a maximum static frictional force that can act will be
${\mu _s}N = \mu mg$
Where N is the normal reaction of the body in the upward direction
mg is the weight of the body
So, on increasing the externally applied force up to $\mu mg$ the amount, static frictional force \[\left( {{\mu _s}} \right)\] will also increase linearly.
So, the maximum value of static friction is $\mu mg$
And the minimum value is 0 which is when no external force is applied.
So, the average static frictional force, in this case, will be $\dfrac{{\mu mg + 0}}{2} = \dfrac{{\mu mg}}{2}$.

Note: After the body has overcome the static friction and started moving, the kinetic frictional force \[\left( {{\mu _k}} \right)\] comes to act and its value is always constant.
If you apply the force on a body and it is not moving it means that the applied force is being utilized to overcome the static friction.