Two tanks
Water enters A, flows into B, and leaves. Change one setting and give the water time to catch up.
Paused at equilibrium. Try doubling the incoming water.
How it works
This is an original teaching model, inspired by the book’s subject. It is not a replica of the Phillips–Newlyn machine or an economic forecast.
Each outlet releases water at a rate proportional to the volume in its tank. With volumes A and B in litres, incoming flow q in litres per minute, and outlet coefficients kA and kB per minute:
A′ = q − kA × A B′ = kA × A − kB × B
An outlet rate of 0.20 per minute means a tank holding 5 litres releases 1 litre per minute at that instant. It does not remove exactly 20% over a whole minute: the flow changes as the tank empties.
The initial state is A = 5 L and B = 10 L. Both outlets pass 1 L/min. Reset restores these volumes, the initial settings and time zero. The drawing and graph use a fixed 0–40 L scale; the ideal equations have no tank-capacity limit.
Play advances one model minute per real second. Changing a setting keeps the existing water. +1 minute advances the paused model. Leaving the tab pauses it. The graph shows the most recent 120 model minutes; CSV includes the full recorded run, up to the 1,000-minute session limit.
A shaped or controlled outlet can approximate this linear rule. An ordinary hole does not generally drain in direct proportion to the tank’s volume. The simulator uses the exact constant-setting solution between changes, with ordinary floating-point rounding.