Is the Kinetic Friction Coefficient Consistent Across Different Modes of Motion?

[badsis]

Hello,

Here is my question:
should the kinetic friction coefficient Fk be the same when the object with mass m is pulled with a constant velocity as the kinetic friction coefficient with the same mass m when it was pushed and released.
The measurements in the second case were taken from the decelerating part of the Force vs. Time graph.

I have several table with data many of which i have calculated myself.
from the formula: Fk = ;mu * Fn
Fk - kinetik friction force,
;mu - coefficient of friction kinetic force
Fn - normal force
in the first case I have:
m = 0.09 kg
Fk = 0.1387 N (the uncertainty here is 17% ! Fk = 0.1387 +- 0.0238)
Fn = 0.882 N
=> ;mu = 0.1573

In the second case:
m = 0.09 kg
Fk = 0.2585 N
Fn = 0.882 N
=> ;mu = 0.2931
here i also have uncertainty in acceleration which I use in order to find Fk.

So, my question is: Should the coefficient ;mu be different in two cases or do I have such a big error?

THANKS!

 

[Kurdt]

The coefficient should be the same. If you are pulling the mass be careful that no component of the force acts upwards since this will reduce the normal force.