how to calculate a negative acceleration?

Question

I\'m implementing the scrolling behaviour of a touch screen UI but I\'m too tired in the moment to wrap my mind around some supposedly trivial piece of math:

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Answer 1

acceleration = (force / mass) 
velocity = (acceleration * time)
(force from user's finger) = acceleration / mass = velocity / time

1. give the view a mass (tweak it until things seem reasonable, and let the user tweak it)
2. identify some new force (drag)
3. give the new force (drag) a magnitude (tweak until reasonable, let the user tweak it)
4. apply the new force to the object and watch it slow down