# Why don't couple forces violate Newton's First Law?

If you have some random object at rest and you apply a couple to it, the net force acting on it is zero. However because a moment acts on it, it starts to rotate.

So you had an object at rest, a net force of zero was applied to it, and it is now moving. Why doesn't this violate Newton's First Law?

• You're still applying a net torque, the rotational analog of a force. Therefore, the object will change its angular momentum (start to rotate).
– Danu
Oct 12, 2013 at 20:31
• @Danu But Newton's Law says the net force has to be non-zero, not torque. The torque isn't zero, but the force is. Isn't that a violation?
– dfg
Oct 12, 2013 at 20:33
• The forces don't both act on the same (part of the) thing, and therefore Newton's laws don't tell us they should cancel out.
– Danu
Oct 12, 2013 at 20:41
• @Danu Buts its a rigid body, so can't the whole thing be treated as one "thing"?
– dfg
Oct 12, 2013 at 20:47
• You can definitely treat the rigid body as a system, but then the only thing that you can conclude from the fact that the net force on the system is zero is that the center of mass won't move. See my answer. Oct 12, 2013 at 20:56