Answer:
For example, when a person walk
Explanation:
The force of friction is a resistive force that opposes the motion of objects. It is produced by the contact between two surfaces: since the surfaces are not perfectly smooth, there are "microbumps" that cause a resistive force in the relative motion of the two surface.
This force is evident, for example, when we try to push a crate along the floor: as we push forward, there is a resistive force acting in the opposite direction, and this force is the force of friction.
The force of friction is in the direction opposite to the motion of the object, so in general it tends to slow down the motion of the object over the surface.
However, there is a particular case in which instead friction helps in accelerating the object.
This scenario is when a person walks: in fact, the foot of the person pushes against the floor (backward), and as a result, the floor "pushes" forward with the force of friction, which helps therefore the person to move forward. In fact, the force of friction is the force that allows us to walk forward: without the force of friction, we could not walk (think about how difficult it is to walk over an icy surface: this is because the force of friction on an icy surface is almost zero!)
