Question: I'm a bit hung up on the idea of forces at either end of a spring being equal, surely by extending the spring you are moving its centre of mass and hence applying a resultant force on it. I get it when the spring is not extending or has already extended, but while it is extending surely the force pulling the molecules apart must be greater than the force pulling them back together and hence there should be a resultant force on each molecule. And then this should accumulate throughout the spring as the force pulling on each molecule gets smaller such that though the fixed end of the spring is in equilibrium, the forces acting on it are less than the force pulling at the other end of the spring.

Answer: 
> 
> I'm a bit hung up on the idea of forces at either end of a spring
> being equal, surely by extending the spring you are moving its centre
> of mass and hence applying a resultant force on it.
> 
> 
> 

Yes.

If the spring is *being extended* then almost by definition its Centre of Mass (in the case of a **massive spring**) is accelerating. *Being extended* here means that one end moves and the other not or than one end moves faster than the other.

This means that acc. Newton's Second Law there must be a *net force* acting on the spring. This in turn means the forces acting on both ends cannot be the same.
The forces are equal at the ends because if they were not one end or the other would move.
If you replace the spring with an elastic string , it is easier to see that the tension in the string is the same all over. And also in the spring (which is a hard kind of curly string) the tension (force) is equal along the length of the spring.