Oscillator

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This little JXSGraph application simulates a damped oscillator. If You can catch the weight with the mouse, you can drag it and the animation restarts after releasing the mouse.

The underlying JavaScript code

var board = JXG.JSXGraph.initBoard('jxgbox', {boundingbox: [-10, 10, 10, -10], keepaspectratio: true, axis: true, grid: false}),
    line = board.create('line', [[0,8], [0,-10]], {visible: false, straightFirst: false, straightLast: false}),
    point = board.create('glider', [-8, -7, line], {name: 'Weight'}),
    isInDragMode = false,
    springHangup = board.create('point', [0, 9], {color: 'black', name: 'Spring', fixed: true}),
    i, numberOfSpringRings = 10, springRings = [];

for(i=0; i<numberOfSpringRings; i++) {
    springRings[i] = board.create('point', [0.5-i%2, function(i) { return function() { return springHangup.Y()-(i+1)*Math.abs((springHangup.Y() - point.Y())/(numberOfSpringRings+1))}; }(i)], {withLabel: false, color: 'black', size: 1});
    if(i>0)
        board.create('segment', [springRings[i-1], springRings[i]], {color: 'black', strokeWidth: 1});
}
board.create('segment', [springHangup, springRings[0]], {color: 'black', strokeWidth: 1});
board.create('segment', [springRings[numberOfSpringRings-1], point], {color: 'black', strokeWidth: 1});

function startAnimation(startY) {
    point.moveAlong(function() {
        var f = function(t, x) {
                var c = 0.1, k = 0.5, m = 1;
                return [x[1], -c / m * x[1] - k / m * x[0]];
            },
            area = [0, 200],
            numberOfEvaluations = (area[1] - area[0]) * 100,
            data = JXG.Math.Numerics.rungeKutta('heun', [startY, 0], area, numberOfEvaluations, f),
            duration = 20 * 1e3;

        return function(t) {
            if (t >= duration)
                return NaN;

            return [0, data[Math.floor(t / duration * numberOfEvaluations)][0]];
        }
    }());
}

function hook() {
    if(!isInDragMode) {
        if(board.mode === board.BOARD_MODE_DRAG) {
            board.stopAllAnimation();
            isInDragMode = true;
        }
    }

    if(isInDragMode) {
        if(board.mode !== board.BOARD_MODE_DRAG) {
            isInDragMode = false;
            startAnimation(point.Y());
        }
    }
}
board.addHook(hook);
startAnimation(-5);