Interactive Self-Study Module: Linear Momentum

Overview:

This module uses screencasts and an interactive simulation to explain how you can calculate forces acting on a fluid as it flows through or over an object. This allows you to calculate the force the fluid exerts on the object. Your retention of material in this module will increase if you write down reasons for your answers to ConcepTests, questions in screencasts, and questions to answer before using interactive simulations, and you try to solve the example problems before watching the screencast solutions. 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 screencasts that describe the forces acting on water that redirect its momentum through a control volume.
  3. Look over the important equations for linear momentum analysis.
  4. Use the interactive simulation to further understand how forces acting on a control redirect the momentum of fluid.
  5. Try to solve the example problems before watching the solutions in the screencasts.
  6. Answer the ConcepTests.
  7. Look at the list of key points, but only after you try to list the key points yourself.
Motivation:
  • Engineers sometimes need to understand how much force a fluid exerts on an object as it flows over or through it. For example, when you hold a garden hose to spray something you have to hand onto the hose or else it will fly around. You need to apply a force with your hands to hold the hose stationary. Another example is that when airplanes land, air is redirected by its thrust reversers to slow the airplane down on the runway.
  • This module is primarily intended for a Fluid Mechanics course.
Before studying this module, you need to be able to:
  • Draw a free-body diagram.
  • Explain the meaning of a dot product.
  • Convert a velocity vector into its x-component and y-component using trigonometry.
  • Draw and work with control volumes.
After studying this module, you should be able to:
  • Identify the direction a force acts on a fluid by examining the magnitude and direction of its velocity entering and leaving a control volume, before doing any math.
  • Identify which forces act externally on a control volume and which forces act internally.
  • Calculate the force acting by an object on a flowing fluid (and hence the force the fluid exerts on an object).