# Can there be force couples in a particle system of equilibrium?

Inertigratus
My teacher said that if a system is in equilibrium, then there cannot be any momentum. I asked, what if there are force couples?
He said that fource couples are disregarded, or they that are not possible in a system of equilibrium. If this is wrong, then I probably don't remember what he said exacty.
Hoping for a better explanation here. Can't there be two opposite forces of equal magnitude affecting the particle and therefor making the particle rotate while the system is in equilibrium?

Mentor
Static equilibrium generally means both translational and rotational equilibrium. Thus, about any point the net moment of all forces would be zero.

But it's certainly possible to have the net force equal to zero while having a net moment, as you point out.

Inertigratus
So to have a force couple in a static equilibrium it means that there also has to be a negative moment the same magnitude as that of the force couple?

Mentor
So to have a force couple in a static equilibrium it means that there also has to be a negative moment the same magnitude as that of the force couple?
Yes. Otherwise it will not be in rotational equilibrium.

phyeinstein_c
@doc atleast during the couple there will be momentum change (angular) or watever...

Mentor
@doc atleast during the couple there will be momentum change (angular) or watever...
Not if the net moment is zero everywhere.

phyeinstein_c
oh yeah.. :P im tubelight

phyeinstein_c
it is also possible that net force (moment/torque ) is 0 but it continues in uniform motion??

Mentor
it is also possible that net force (moment/torque ) is 0 but it continues in uniform motion??
Definitely.

phyeinstein_c
ohkk... :)