Question

Then it is called

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

Answer 1

It is called force of friction

Step-by-step 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.