21. True or False: Static friction prevents you from slipping when you walk on a sidewalk.

Yes it is true because static friction help us to make grip between the foot and the ground surface

Explanation:

Yes, as we know that the friction between two surface is of two types

1) Static friction

2) Kinetic friction

When two surface moves relative to each other then the friction force between two surface is kinetic friction and it always opposes the relative motion of two rough surface

While when one surface has tendency to move with respect to its contact surface but if it can not move on it then in this case it is static friction

So relative motion is not possible in this type of friction

So static friction helps to prevent slipping