Can you accelerate a car on a frictionless horizontal road by putting more petrol in the engine? Can you stop a car going on a frictionless horizontal road by applying brakes?
What is the average friction force when a person has a usual 1km walk?
Answer
Zero.
As $<\vec{\text{a}}>=\frac{\vec{\text{v}}_{\text{f}}-\vec{\text{v}}_{\text{i}}}{\text{t}}=\vec{\text{0}}$ as men starts with zero velocity and ends with zero velocity.
For most of the surfaces used in daily life, the friction coefficient is less than 1. Is it always necessary that the friction coefficient is less than 1?