Vertical 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 vertical springs anywhere in the simulation window. With the Select tool, you can move the spring.

To attach a body element to a spring, click on the Body Element button and then click anywhere on the spring. When attached to the spring, the body has a square, rather than round, shape. You cannot resize the body, nor can you represent the body with an image from the clipboard. You can move the body to compress or expand the spring, as indicated in the figure. But you cannot remove the body from 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). However, in the Properties Box, you can turn off (or on) automatic equilibrium mode. You can also set the spring's rigidity (the spring constant). The default rigidity is 1.0. For this rigidity, a one kilogram body displaced one pixel from the spring's equilibrium position experiences a force of one newton.