This simulation deals with Atwood's machine, a classic situation in which Newton's second law can be applied to determine the acceleration of two masses connected by a string that passes over a pulley. To begin with, the mass of the pulley is set to zero, allowing you to focus on just the blocks and the string. Note that the system is initially held at rest by applying a downward force to the lighter block.

Here are some things to investigate:
 1 With the mass of the pulley set to zero, give one block a larger mass than the other. Do you agree with the free-body diagrams shown in the simulation? By applying Newton's second law to each block, calculate the acceleration the system will have when it is released from rest. Does your calculation agree with the value obtained by the simulation?
 2 In general, the acceleration of the system is less than g, the acceleration due to gravity. If you want the acceleration of the red block to equal g, what must the mass of the blue block be set to? When the acceleration equals g, what is the tension in the string?
 3 Set the mass of the blue block to 4 kg. Adjust the mass of the red block until the magnitude of the acceleration is minimized. Adjust the mass of the red block again until the magnitude of the acceleration is maximized - given the limits on the mass of the red block, should the red block be as light as possible or as heavy as possible?
 4 With the mass of the pulley set to zero, and one block heavier than the other, note the acceleration. Now, predict what will happen to the acceleration if the pulley is given a mass. Does your prediction agree with what the simulation shows? Note that the effect of the pulley will be studied in some detail in a later simulation.