Horizontal Spring: This tool subjects an attached body element to an elastic force that is proportional to the compression or expansion of the spring. Place between one and three horizontal springs on tracks only. The spring initially faces right (as in the button icon), but you can change that in the spring's Properties Box by unchecking the box labeled reverse. With the Select tool, you can move the spring along the track.

To attach a body element to a spring, click on the Body Element button and then click anywhere on the spring. By default, when you attach the body (or change the properties of either the body or the spring), the spring adjusts itself to an equilibrium position (adjusted for gravity when present, but not for other forces).

During simulations (Run mode), bodies on horizontal springs break free of the springs if they move past the blue joints that connect the track sections and terminate the left and right sections. Conversely, bodies moving along tracks can attach themselves to springs, provided the spring does not lay on a blue joint.

In Stop mode, you can detach bodies from springs by moving them past blue joints, but you cannot attach bodies to springs except by placing them.

In the Properties Box, you can set the spring's rigidity (the spring constant). The default rigidity is 5.0. For this rigidity, a one kilogram body displaced one pixel from the spring's equilibrium position experiences a force of five newtons.