Go and spawn a combine thumper prop. The path is models/prop_combine/combinethumper001a.mdl from HL2.
Pick it up with your physgun, and try to fling it across the map. You’ll find that you can’t - if you try, the prop stops moving as soon as you let go of it, and slowly falls to the ground.
Both types of thumpers do this. There may be other built-in models that do this, but I haven’t found them.
Does anyone know what causes this? Perhaps some missing physics information? The reason I’m asking is that my new model also has this behaviour.