Confusion about rotation of a beam due to a couple moment and Newton's 2nd Law I'm struggling with the following concept.
If we have a uniform beam with a single pivot at its centre, and we apply two forces in opposite directions on opposite ends creating a couple moment, then Newton's 2nd Law states that (because the forces cancel out) there will not be any motion (or at least transnational motion?)
But, of course, there will be rotational motion, as there is a net moment on the beam. I am struggling to connect the idea of Newton's 2nd Law and the forces together with rotational motion, as my intuition keeps telling me that there should not be any motion whatsoever as the forces cancel out.
Does this have to do with the internal forces of the beam? Is it because, in the transnational case, we are analysing the beam as if it is a single particle, but in the rotational case, we are analysing it as if it is a "line" of multiple particles, and therefore (if we look at the particles on either end of the beam) the net force is in a direction so as to cause rotation? If this is the case, then what causes the force on the particles on either end that accelerates the particle "inwards" in the normal direction and causes the signature "rotational" motion instead of the particle just accelerating off into space?
I hope I've gotten across what I'm confused about! :P
 A: 
then Newton's 2nd Law states that (because the forces cancel out)
there will not be any motion (or at least transnational motion?)

For a force couple, there will not be any translational motion of the center of mass but there will be rotational motion.

But, of course, there will be rotational motion, as there is a net
moment on the beam. I am struggling to connect the idea of Newton's
2nd Law and the forces together with rotational motion, as my
intuition keeps telling me that there should not be any motion
whatsoever as the forces cancel out.

For translational motion, Newton's second law is
$$F=ma$$
For rotational motion, Newton's second law gives us
$$τ=Iα$$
Where
τ = torque and is analogous to force $F$
$I$ = moment of inertia and is analogous to mass $m$
$α$ = angular acceleration and is analogous to linear acceleration $a$
A force couple consists of  two equal, but opposite parallel forces $F$ separated by a distance $d$. So the net force is zero (no translational motion) but the net torque or moment about the center between the forces is $Fd$.

Does this have to do with the internal forces of the beam?

No. It has to do with the motion of the beam as rigid body. On the other hand, moments on a beam are associated with internal stresses in the beam (a mechanics of materials problem).

Is it because, in the transnational case, we are analysing the beam as
if it is a single particle, but in the rotational case, we are
analysing it as if it is a "line" of multiple particles, and therefore
(if we look at the particles on either end of the beam) the net force
is in a direction so as to cause rotation?

From a dynamics perspective we are analyzing the beam as a rigid body. However, force couples do cause internal stresses in a beam when we analyze those stresses as a mechanics of materials problem.
Hope this helps.
