A slider in a Manipulate will display its current value, expliticly, if it has the option Appearance -> "Labeled"
Manipulate[
moment = w*fos*l^2*0.125,
{w, 1, 500, 10, Appearance -> "Labeled"},
{fos, 1.5},
{l, 1, 16, 1, Appearance -> "Labeled"},
FrameLabel -> "moment"
]