Question

Does a constant speed indicate dynamic equilibrium?

Answer 1

Not necessarily.

Explanation:

To have a constant velocity indicates that an object is not undergoing acceleration, and therefore there cannot be a net force acting on the object. Then `F_"net"=0`, and this is the definition of equilibrium. As the object is still moving, we would call this dynamic equilibrium. Here I'm thinking in terms of linear/translational motion.

However, I think of circular motion where a technically comes into play. The question posed is whether a constant speed necessarily indicates that an object is in dynamic equilibrium. In circular motion, an object might be following a circular path at a constant speed, but the velocity is necessarily constantly changing direction due to centripetal acceleration caused by the net centripetal force which is necessary to keep the object moving in a circular path. Therefore, `F_"net"!=0` and the object is not in dynamic equilibrium.

There is a concept of rotational equilibrium, which indicates that the net torque on the object is zero, i.e. the angular acceleration of the object is zero. However circular motion does not always mean rotational motion, and as this was not the original question, I will not consider this case.