Interactive Self-Study Module: Conservation of Mass


This module uses a screencast to explain how you can analyze the mass flow rate of fluids into and out of a control volume. This allows you to calculate things like volumetric flow rate, average speed of a fluid through a nozzle, and the rate at which a vessel is filling up or draining fluid.

Your retention of material in this module will increase if you write down reasons for your answers to ConcepTests and you try to solve the example problems before watching the solutions in the screencasts.

We suggest using the learning resources in the following order:

  1. Attempt to answer the multiple choice ConcepTest and solve the example problem before watching the screencasts or working with the simulations.
  2. Watch the screencast that describes the continuity equation.
  3. Look over the important equations.
  4. Try to solve the example problems before watching the solutions in the screencast.
  5. Answer the ConcepTests.
  6. Look at the list of key points, but only after you try to list the key points yourself.
  • It is important to be able to draw meaningful control volumes and calculate the rate at which mass flows into and out of a control volume. For example, it is important to understand how the speed of a fluid changes as it flows through a nozzle, or how the volumetric flow rate of air changes when it is heated up down a length of pipe.
  • This module is primarily intended for a Fluid Mechanics course.
Before studying this module, you need to be able to:
  • Explain the meaning of a dot product.
  • Convert a velocity vector into its x-component and y-component using trigonometry.
After studying this module, you should be able to:
  • Draw and work with control volumes.
  • Simplify the macroscopic continuity equation for a specified control volume.
  • Calculate the average velocity of a fluid flowing through a constriction.
  • Calculate the rate at which mass is accumulating or depleting within a control volume.