Now think! When a child pushes a toy car on the floor, why does it stop? Should he keep moving forever?
The answer is no! The cart would only continue in uniform rectilinear motion forever if the resultant forces acting on it were zero. But it is not. There is a force that the ground exerts on the body, parallel to the ground and contrary to movement. This force is called the frictional force.
The frictional force depends on the texture of the surfaces that are in contact. The more polished, the less resistance a surface offers to the movement of a body moving over it.
This frictional force also depends on the force that the moving body makes perpendicular to the surface. The higher this force, the greater the frictional force.
Sometimes the frictional force acts on the body without it being in motion. For example, a body rests on an inclined plane because the frictional force is preventing that body from sliding down the plane. The force of friction is always contrary to the tendency of movement of the body.