**General comments**

In this lecture we started with a review of the material discussed last lecture
via a concept question (Q1). This was
intended to reinforce that that thrust is primarily a balance between momentum
flux into an engine and momentum flux out of an engine. We then discussed the
integral momentum equation for a fixed mass in a reference frame which is accelerating
relative to an inertial reference frame. The equation is identical to that used
in fluids with the addition of a term to account for the acceleration of the
reference frame (which you have seen in Dynamics before). We then discussed
in some detail the calculation of momentum flux using the integral momentum
equation. Please review the concept question (Q2).
It is important that you understand how to apply this. Note that there was some
concern expressed about the sign obtained in this example. Since I specified
that you were to find the momentum flux into the surface, then the sign **should
be positive** (since the flow is clearly into the surface). But to avoid
confusion, I just changed the wording of the problem
to say "determine the momentum flux across the surface". Thus,
if the calculation results in a minus sign, then the flow is opposite to the
outward normal (i.e. into the surface); if the calculation results in a positive
sign the flow is out of the surface.

**Responses to 'Muddiest Part of the Lecture Cards'**

(4 respondents--must be the last lecture before spring break)

1)* So what if we want the
total momentum flux for this problem?* (1 student)
The total momentum flux is just the magnitude of the vector composed of the
x, y, and z components.

2)** Donuts = more attentive
students **(1 student). I'm on it.

3) * No mud* (2 students). Good.