Math Help, Weight added to "throw" an image

Hello,
I am trying to acheive the following effect and need some help getting the math down.

I have a 360 degree panoramic photo. I set it so that you can drag the photo left or right to appear as though it will keep spinning around.

Right now i am using an onMouseMove to move the photo relative to the _xmouse.

I would like to add a function that will give the photos weight, so the mouse can kind of throw the image.

so, when you drag the image and let go, it has weight and will slow down gradually as though the pictures was slid accross the screen. Thanks for any help. i have seen throw ball codes that are probably similar, but couldn’t get them to work with this applications.
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