Common mistakes on HW2

Hi class-

Since you received HW2 back today, please take a few minutes of your time to go over the problems and pinpoint those places where you made mistakes. Just a small time investment reviewing each returned problem set will enhance your success in the course and ultimately save you many more hours later on when you are preparing for exams.

There were a couple of places where you seemed to have problems. In particular:

1) It's important to write down the equations first, before you fill in the values. This greatly reduces the chance for mistakes.

2) Some of you are having problems with the difference between lbf and lbm. Remember, lbf is a measurement of force. It has units of [mass*distance*time^(-2)]. On the other hand, lbm is a measurement of [mass]. You cannot say 1 lbf = 1 lbm, because there are differences of the basic units here. Instead, you can relate a lbf to a lbm times an acceleration, which has units of [distance*time(-2)]. However, you have to keep in mind that 1 lbf is not equal to 1 lbm * ft/s^2. Instead, 1 lbf = 32.17 lbm ft / s^2. You can just think of this as a unit conversion, just as if you would convert one kind of pressure to another.

3) There was some confusion about problem number 6 regarding cost scalings. Please pay particular attention to the solutions here.

4) For problem 7, remember that the universe as a whole is an isolated system (there is nothing else!). Therefore, according to the first law, the energy of the universe always remains constant.

-MSS