  We determine the invariant expression of the force density that the
electromagnetic field exerts on dipolar matter and construct the non-symmetric
energy-momentum tensor of the electromagnetic field in matter which is
consistent with that force and with Maxwell equations. We recover Minkowski's
expression for the momentum density. We use our results to discuss momentum
exchange of an electromagnetic wave-packet which falls into a dielectric block.
In particular we show that the wave-packet pulls the block when it enters and
drags it when it leaves. The usual form of the center of mass motion theorem
does not hold for this system but a modified version of the theorem which
includes a spin contribution is shown to be satisfied.
