September 25, 2011

Given distance and friction, calculate the initial velocity to stop at the distance target

While simulating iOS scroll views in javascript, I needed to find the initial velocity to send the scroll content back into position if you drag it out of bounds. Since the in-bounds scroll view inertia was already working with a given friction, I wanted to use this to run the calculation. After some brute-force experimenting, I came up with the equation to calculate this velocity:
// set up vars
var curPos = 0;
var distance = 2000;
var friction = 0.8;
var velocity = distance / ( friction * ( 1 / ( 1 - friction ) ) );

// in timer:
    velocity *= friction;
    curPos += velocity;
The curPos value will ease into the target distance, no matter what you set as the friction and distance. Friction must be a decimal between zero and one.

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