Friction forces

Question

A tired worker pushes a heavy (100-kg) crate that is resting on a thick pile carpet. The coefficients of static and kinetic friction are 0.6 and 0.4, respectively. The worker pushes with a force of 600 N. The frictional force exerted by the surface is aproximatly?

Answer 1

Anita,

 

This looks very similar to another problem we looked at earlier tonight.  First, decide if the crate is moving or not by comparing the applied force to static friction. 

 

f_s = μ_smg = (0.6)(100)(9.8) N = 588 N < 600 N

 

It is moving, and therefore it is opposed by kinetic friction.

 

f_k = μ_kmg = (0.4)(100)(9.8) N = 392 N