Fill in the empty spaces with motion/action blur in this simple tutorial.

We’re going to make it look like the ball on the far right of this picture is moving, rather than the crisp still image of the movement.

Use one of the Lasso tools to select around the right-hand ball and string.

Copy and paste it into a new layer.  Trim around the ball and string to make it cleaner.

Separate the ball from the string, on two different layers.

Begin to duplicate the ball and string separately, placing them from left to right, towards the furthest ball.

Space them a little wider at first, then slightly closer together as you near the final ball. Be sure you are layering them in appropriate order.  Be sure to put one over the last ball, placed where the background image is, so it continues to cascade in order.

If you have not, merge the strings to each of their corresponding balls.  Now, we’re going to make them all transparent in different percentages. From the left: 20%, 35%, 50%, 75%, 100%.

In a new layer over all of the layers, draw a shape that is rounded (doesn’t have to be perfect) at the right side and swoops and squares off at the end.  Fill it with white and use your Brush tool with the color black and a soft round brush.  Fill some of the area in with black from the left to the right.  Change the Layer’s Blending option to Linear Dodge.

Drag the layer and place it below the other layers and above the background in the layers window.  Use an Eraser and carefully erase some of the edges, leaving so it appears  just along the edges of the one that is swinging.

Duplicate the uppermost ball once more.  We don’t need the string in this one so if you still have the string attached from merging the two together, you can erase the string altogether.

Select the Smudge tool and yes, a soft round brush to go with it. Be sure the Mode is Normal and the Strength is at 30%. Now, on the newest ball layer that is over the top of everything in the layers, click and drag along the swinging path.  You’ll have to do this a few times to get it to reach where it began swinging initially.

Using the Eraser, Soft Round brush and 15% Opacity, scale the brush up in size and then click a few times on the new blurry/streaming ball. Adjust it as you see fit.

The last and final touch is just making the ball that the swinging one bounced off of, jiggle just a little bit.  Trace around that ball, copy and paste into a new layer.  Line it up with the preexisting one, then offset it by a few pixels, but not too much.

Make it transparent either using the eraser or the layer’s opacity settings.  Do the same with the string.

There ya go! All done.