Question

Then it is called

Answer 1

Answer:

It is called force of friction

Explanation:

The force of friction is a force that acts between two objects whose surfaces are in contact with each other.

Consider the typical case of an object sliding along a certain surface. There are two types of frictions:

- Static friction: this is the force of friction that acts when the object is not in motion yet. If you push the object forward with a force F, the object will not move immediately, but it will "oppose" to this motion with a force of static friction exactly equal to the push applied:

$F_f = F$

However, this force of static friction has a maximum value, which is given by

$F_{max} = \mu_s N$

where

$\mu_s$ is the coefficient of static friction

N is the normal reaction exerted by the surface on the object

So, when $F$ becomes greater than $F_{max}$, the static friction is no longer able to balance the push applied, and the object will start sliding forward.

- Kinetic friction: this is the force of friction that acts when the object is already in motion. Its magnitude is given by

$F_f = \mu_k N$

where

$\mu_k$ is the coefficient of kinetic friction, and its value is generally smaller than $\mu_s$. The direction of this force is also opposite to the direction of motion of the object.