Eddy braking · Lenz's law simulation

Drop a magnet down a copper tube and it crawls — taking seconds where free fall takes a flash, with nothing touching it. Does the magnet keep accelerating like any falling body, or does the braking it induces settle it at one steady speed?

▶ Run the simulationSee the measured result

How the lab tests it

Release two identical bar magnets at once: one down a copper pipe, one in free fall beside it. The magnet's own moving field induces eddy currents in the pipe wall (Lenz's law — they oppose the change), giving a velocity-proportional brake F = −b·v, so M dv/dt = Mg − bv. The lab traces v(t) for both. A real NdFeB magnet (m = 0.55 A·m²) is also dropped through a real thin-walled copper pipe with 2% reading noise, using b = (45/1024)μ₀²m²σw/a⁴ for a point dipole on the axis of a thin tube.

What it checks

TERMINAL VELOCITY — the brake grows with speed until it exactly balances gravity, so v saturates at v_term = Mg/b along v(t) = v_term(1 − e^{−t/τ}) (τ = M/b): the free magnet's v = g t runs off the top of the chart while the braked one flattens onto its asymptote. At terminal velocity kinetic energy stops growing, so ALL the lost gravitational PE becomes I²R heat in the tube; and measuring v_term ≈ 0.2 m/s inverts to recover copper's conductivity σ ≈ 5.96×10⁷ S/m