F, NForce–extension
This 2D side-view simulator hangs a mass from a spring fixed to a ceiling mount and traces its real force-extension curve. Drag the extension slider (or press Auto Stretch) to pull the spring slowly and watch the force climb in a straight line while inside the elastic region — release at any point here and the spring snaps fully back. Push past the elastic limit and the curve bends into a plastic, strain-hardening zone where the spring keeps a permanent set after release, and pushing further still leads to thinning and fracture. Adjust the spring constant and mass, or press Drop Mass to watch genuine F = -kx dynamics — gravity, the spring's restoring force and light damping — decide whether the mass settles gently or overshoots straight past the elastic limit on its first swing.