The Atwood Machine in a Fluid model shows two masses connected by strings and pulleys immersed in a water tank. Each mass is acted on by gravitational, buoyant, and drag forces. Users can change density of the left and right mass using input fields and can drag either mass to change the initial conditions.  Note that the floor of the tank produces a normal force that limits the motion.
 
If the water is removed the tank,  the model behaves like a traditional Atwood machine.  Adding water changes the dynamics in many ways.  Because the buoyant force varies as the mass is submerged, the system oscillates anharmonically about an equilibrium point that depends on the mass densities.

The Atwood Machine in Fluid model was created using the Easy Java Simulations (EJS) modeling tool.  It is distributed as a ready-to-run (compiled) Java archive.  Double clicking the ejs_mech_fluid_AtwoodMachineInFluid.jar file will run the program if Java is installed.

Please note that this resource requires at least version 1.5 of Java.