Then it is called

5. if a contact force resists the sliding
motion between two objects that are
touching,

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:

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

where

is the coefficient of static friction

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

So, when becomes greater than , 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

where

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

an electric circuit