In the collision of a particle (A) with a system of bonded particles (B–C), only the energy in the center of mass reference frame is available for breaking the bond. This is because some of the initial energy must be used in conserving the momentum of the system. This article describes a simple air‐track experiment illustrating this well‐known fact. Two carts on a horizontal air track are bonded together by small magnets; a sudden impulse is provided to one of the carts by a falling weight. A feature of the experiment is the direct determination of magnetic bond energy by numerical integration ofFdx. For cases where the energy in the center of mass is sufficient to break the magnetic bond, the ensuing motion of the system can be described in terms of an effective mass.