Where's the third force in a hanging mass and spring

[Wattever]

Sorry if this is a stupid question, I haven't studied mechanics in a while and suddenly I'm confused about this.

A mass m is hanging at the end of a spring. Forces on the mass: mg downwards and F upwards. Forces on the spring: mg downwards and a force upwards exerted on the spring by the ceiling. Is that correct? If so, where are the two forces that are equal in magnitude and opposite in direction? Shouldn't it be that if the mass pulls the spring with force P (=mg), the spring pulls the mass with an equal force P (so F=P)? I know this is not so though, since the mass does accelerate.

 

[kcdodd]

The force on the mass from the spring is equal/opposite to the force on the spring from the mass. The forces are on the two separate objects, not the same object. The force on the spring is not mg, that is the force on the mass.