Place the cylinders into the vertically held Plexiglas tube.  When the green ends are facing each other, the system is simulating two electrons approaching each other, and they are always repulsive, independent of distance.  When the orange ends are facing each other, the system is simulating two protons approaching each other.  They are repulsive at large distances, but at small distances the repulsive force goes to zero and they become strongly attractive.  This is the analogy of the electrostatic repulsion that dominates at large distances, and of the strong attractive component of the nuclear force responsible for the energy released in fusion (The above is excerpted from the Kashy and Johnson reference below).

Place one of the cars near the middle of the air track with the magnet on the car facing the oncoming car.  As you gently collide the cars they will repel each other without touching.  If you collide the cars at greater velocities the magnetic repulsion will be overcome and the cars will touch.