With the Background Copy selected, go to Layer > Smart Objects > Convert to Smart Object. Then go to Filter > Blur > Motion Blur. Set the Angle to zero, and play with the distance until it looks realistic. Click OK, then turn Layer 1 on.If you need to adjust the blur, double-click on the Motion Blur Smart filter to redo it.
The most complicated of this tutorial is making the selection. We used the Quick selection tool, followed by the Brush tool, in Quick Mask mode. But other methods could work, too. If the subject you want to separate is, on the whole, a different color from the background, try using select > Color range. Hold down the shift key and click on multiple points of your subject to get a selection. The Magic Wand is usually inferior to the newer Quick selection tool, but it will occasionally work when Quick selection has failed you.