Algodoo

Works well. W could be a toggle.

It could but I don't know any reliable way to script a toggle button.

In green box onKey:
keys.isDown("w") ? {
scene.my.springLength == 1.2 ? {
scene.my.springLength = 0.5
} : {
scene.my.springLength = 1.2
}
} : {}

in black circles' postStep:
scene.my.springLength == 0.5 ? {
collideSet = 1;
color = [0, 0, 0, 1]
} : {
collideSet = 0;
color = [0, 0, 0, 0.5]
}

in spring postStep:
length = scene.my.springLength

how do you get something to go towards mouse?