Faraday’s law. The part of the disc under the magnets is moving through the magnetic field. The flux through any piece of metal that is entering the field is increasing, and for a piece that is leaving it is decreasing, so EMFs are induced in the disc and currents (eddy currents) flow in it.

Lenz’s law. The currents flow in the direction that opposes the change. On the side where the metal is entering the field, the current makes a field that points against the magnets’ field, and where the metal is leaving, it makes a field that points the same way. The two loops of current meet in the middle, where they flow the same way, and there the field pushes on the current so that the disc is slowed down.

EMF ∝ Bω, current ∝ Bω, braking torque ∝ B × current ∝ B²ω

What that means. The braking is proportional to the speed, so the speed falls exponentially, and it is proportional to the square of the field: a field 2.5 times as strong brakes the disc 6.25 times as hard. There is no braking at all when the disc is at rest, because there are no eddy currents. Flipping the magnets reverses the currents but not the braking.

Holding the motor on. The disc speeds up until the braking equals the push of the motor. That steady speed is the motor’s acceleration divided by the braking constant (kB²).

Where the energy goes. The kinetic energy of the disc is turned into heat in the metal, with no contact. This is how the brakes on some trains and roller coasters work.

The braking is shown as an angular deceleration (rad/s²), not a torque, because the model has no moment of inertia. The disc has no friction, so it keeps turning when the magnets are off.