Recently, many more problems with changing height, velocity, with or without friction and so on,
as you can see that during the class we have no more time left to do more problems with numbers in
detail. I mentioned in my syllabus at the very beginning that there is no way to learn physics by
following solved examples. You need to understand and be able to follow and learn the METHODS.
Understand equations and know when to use which.
Understanding is the key. If you have a very hard time with the HW, I really would like to spend more
time with you after the class to help you. Please see me after the class to schedule a time.
To start, let's do the following first.
In chapter 7 and 8, the most important things we learned was:
1, work changes kinetic energy. It is always true that Wtotal = Kf- Ki = 1/2 m vf^2 - 1/2 m vi^2
2, spring force and mg are special forces (conservative forces) when they do work they transfer
energy between KE and PE. As a result, when Wnc= 0, we have a happy joint account of Kinetic
Energy + Potential Engergy. That's when there is no friction or external forces does work,
Kf+Uf = Ki +Ui
3, If forces other than spring force or mg do any work, Wnc is not equal to zero,
K+U will change by the amount of Wnc
Kf+Uf = Ki +Ui + Wnc
UNDERSTAND all of the above first.
FOR HW 7b and 7c,
In some problems,
you were told vf and vi, so that you know kinetic energy change Kf-Ki,
what causes that change? Total Work.
Wtotal = Kf- Ki Total work = W done by mg + Work done by resistance etc…
Wmg + Wf =Kf -Ki , you know Kf, Ki and can calculate Wmg, you will find Wf.
On the other hand, It is also always true that
Kf+Uf = Ki +Ui + Wnc
Work done by mg is not Wnc, Only resistant force contriute to Wnc
because you know vf, vi, hf, hi, you know Kf+Uf = Ki +Ui and can find Wnc, which is euqual to work
done by resistance.
In some other problems, You were told vi, hence you know Ki.
All you need to do is to find work done by each force, and add them together as Work total.
Then, using Wtotal = Kf- Ki you can solve Kf, hence vf