why the rotation is not smooth?

Hi mate,

Plain and simple: Flash / Animate cannot Classic / Motion tween small rotations over longer spans of frames.

This has been a known issue for decades.

In your case you have 0° to -3.5° spanned over 50 frames.

If you use classic tween instead and convert all frames to keyframes you can check the rotation values for each frame and it all will become quite obvious.

You can try manually setting values such as -0.1° or -0.2° and you'll see that they get rounded to zero.

Flash just can't do this kind of thing.

It has nothing to do with your frame rate or the size of bitmaps as the ACPs have suggested. Even if you had just a vector rectangle you would face the same problem.

The problem is mathematical and how Flash rounds and calculates rotations. -3.5° over 50 frames just does not give it enogh increments to work with and that is why you end up with a jerky movement that is literally one step every 6 frames or so.

This smooth rotation just cannot be done with a Classic or Motion tween.

Now here is what you CAN do: You break apart the bitmap and then Shape Tween it inside your container.

Shape Tweens don't have this problem and everything works as smoothly as you need it.

Here is your fixed file: https://drive.google.com/file/d/1aNKS6FSYldy-aIxek9-_a7qekxvf5Vj7/view?usp=sharing

Good luck!