In Explicit Dynamics, you want to ramp the velocity up from zero to some value. If you apply an instantaneous velocity at t=0 then you are introducing a huge shock into the structure. Don't do that because it will create a lot of noise in the results. You can request the reaction force to the velocity boundary condition.
If you apply a force, it will create an acceleration on the mass. You must have enough mass to avoid creating a huge acceleration. If you don't apply enough force, the motion will stop when the resistance of the structure pushes back with an equal force.
There are several output results concerning Energy. You will have to do some research on this site. Others have asked this question. Here is a Google Search result for this question:
Unfortunately, the post that had a copy of this model did not get transferred when the site was upgraded.