When people walk, their feet should move backwards (push the ground). At this time, the direction of friction is opposite to the relative movement direction of their feet, that is, the direction of friction is forward, consistent with the direction of people walking, which is helpful to people walking, rather than hindering them.

If people want to move forward, they must be pushed by external forces. Since people are moving forward, the external force is naturally moving forward, and the external force here is friction.

When people walk, their feet push backwards, and the ground gives people forward friction. This is an example of the interaction of Newton's third law. Like swimming and rowing, both hands and paddles drain water backwards, thus gaining forward power.

The friction direction of the extended data must be opposite to the relative motion or relative motion trend of the object. For example, an object moving in a circular motion on a disk is subjected to friction perpendicular to the direction of motion, but the direction of friction is opposite to the direction of relative motion between the object and the disk.

Friction is divided into static friction, rolling friction and sliding friction. When an object slides on the surface of another object, there is friction between the contact surfaces that hinders their relative movement. This friction is called sliding friction. Sliding friction is related to the roughness and pressure of the contact surface. The greater the pressure, the rougher the contact surface of the object and the greater the sliding friction.