Linear Momentum: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
ConcepTest 1 Answer  — for more discussion see this screencast
ConcepTest 2 Answer — for more discussion see this screencast
Key points from this module:
  • External forces acting on the fluid cause the momentum of the fluid leaving the control volume to be different that the momentum entering the control volume. This means that the fluid either slowed down or sped up, changed direction, or both.
  • When you draw forces on your free-body diagram, make sure they are external forces. These are forces acting on the fluid. They do not represent the forces that the fluid exerts on an object.
From studying this module, you should now 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).
Prepared by Jeffrey Knutsen, Department of Mechanical Engineering, University of Colorado Boulder