👟 Foot-Ball Impact Lab: Effective Foot Mass, Restitution and Contact Force in a Spring-Damper Kick — 2D Cross-Section

A lumped leg of effective mass M strikes a football through a spring-damper contact whose damping reproduces your chosen restitution e exactly. See the contact force curve, the impulse, the energy lost and why the ball speed is (1+e) M v / (M+m) and never more than (1+e) times the foot speed.