How Do You Calculate the Coefficient of Kinetic Friction for a Moving Crate?

[Mebmt]

Homework Statement

Horizontal force of 210 N used to push 20 kg crate over 4 m with constant velocity. Looking for coefficient of kinetic friction.

Homework Equations

a=0
Work done by friction force is:
Net force=ma
Fapp-Fk=ma
210-Fk=0
Friction force = 210

The Attempt at a Solution

friction force=(muk)mg
210=(muk)(20)(9.8)
muk=1.07

Where did I go wrong?
Scott

 

[PhanthomJay]

Homework Statement

Horizontal force of 210 N used to push 20 kg crate over 4 m with constant velocity. Looking for coefficient of kinetic friction.

Homework Equations

a=0
Work done by friction force is:
Net force=ma
Fapp-Fk=ma
210-Fk=0
Friction force = 210

The Attempt at a Solution

friction force=(muk)mg
210=(muk)(20)(9.8)
muk=1.07

Where did I go wrong?
Scott

assuming a horizontal surface, looks OK. muk can be greater than 1 for certain rough or rubbery surfaces.