Question

Ive got a bit stuck figuring it out for the negative direction? it must be really simple, but just cant seem to get it!

x = current x position

dir = direction of motion on x axis

if (tween == 'linear'){

    if (dir == 1) {

        x += (x / 5);
    }

    else if (dir == -1){

        //what here??
    }
}

Answer 1

What’s missing here is that you need to consider deviations from the starting point, not x=0 (and also consider the sign of the direction as well, which others are stating correctly). That is, if your starting point is x0, your equation should be more like:

x += (x-x0)/5

Here’s the figure for motion in the positive and negative directions (note that position is on the vertical axis and time on the horizontal)

And here’s the Python code. (Note that I’ve added in a dt term, since it’s too weird to do dynamic simulation without an explicit time.)

from pylab import *

x0, b, dt = 11.5, 5, .1

xmotion, times = [], []

for direction in (+1, -1):
    x, t = x0+direction*dt/b, 0  # give system an initial kick in the direction it should move
    for i in range(360):
        x += dt*(x-x0)/b
        t += dt
        xmotion.append(x)
        times.append(t)

plot(times, xmotion, '.')
xlabel('time (seconds)')
ylabel('x-position')
show()