Friction coeffecient between two stacked blocks moving at constant velocity So I came across a problem, it says that there are two masses, $m_1$ and $m_2$, stacked on top of each other, and they are moving at a constant speed. There is also friction between the two blocks, with coefficient $\mu$. It gives us the values of $m_1$ and $m_2$, and it asks us to find the coefficient $\mu$. Is there even friction? If so, how do I find $\mu$?
 A: As you see in the Free Body Diagram, the equilibrium equations are:
$$
F_y=N-m_2g=0 \to N=m_2g \\
F_x=f_r=0 
$$
As you see, if your assumption is the block is moving at a constant speed, the friction force $f_r$ over him must be zero.

If $m_1$ start moving from $v=0$ with a positive acceleration, the friction force $f_r$ will be nonzero, since you are accelerating. only due to the action to the friction.
But once you are at a constant speed $v=v_0$, that friction dissapears. The friction coefficient, stating the maximum allowable value of that friction force, $f_r^{max}=uN$, is still there. But the friction is zero.
Finally, if the mass $m_1$ starts stopping its movement with a negative acceleration, then, the friction force will reappear, pointing against your movement, and if the acceleration $|a|$ is equal or over $uN$, it will increase up to its maximum allowable value $uN$.
Obviously, if $|a|$ is greater than $uN$, then the friction will be insufficient, and you only will accelerate at $uN$, slow than $m_1$, and you will fall from $m_1$
