A student is testing magnetic couplings between toy plastic canal barges floating in a trough of water.
She connects several barges together with magnetic couplings.
She pulls the line of barges using a small electric winch at a constant speed, as shown in Figure 1.
She keeps adding more barges to the back of the train and pulling them.
When there are eight barges in total, the magnetic coupling between barge C and barge D breaks, and the train splits as shown in Figure 2.
Explain why the train of barges splits apart when the number of barges is increased.