Question

A 15-N mass hangs from a spring that is attached to the ceiling. Assuming the mass is stationary, what is the value of the spring force?
Is the answer 15 N?

Answer 1

First - you should question the setter of this question. 15N is a WEIGHT (i.e. a Force) NOT a mass.

The weight of a mass under earth's gravity is mg so it's mass is \[\frac{ 15 }{ g }\] kg

But to answer your question

If the mass is stationary then all the forces acting on it must 'cancel out' i.e. no resulting force.

There are only 2 forces acting - gravity creating a force of 15N and the spring force.

You should note that force is Vector - i.e. it has both size (magnitude) AND direction.

In order to cancel out the spring force must have the same magnitude, but the opposite direction to the weight.

So - your answer is essentially correct - but you should indicate the direction of the force, as well as its magnitude

Answer 2

Thank you for such a detailed reply(: @mrnood