# Struggling with Newton's Second Law Equations

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1. Oct 11, 2014

### TehCylons

Hello,

I am a second year undergrad student majoring in Astronomy and Computer Science. I am having a hard time with my physics courses for the following reasons:

1) SmartPhysics is the text we use (if you can call it a text...). Basically it is an online HW and "pre-lecture" system that allows students to watch a 15 minute lecture that covers ALL topics of a unit. The textbook is very short and is verbatum what the pre-lectures give you (with few exceptions). My instructor does not lecture. We are told to watch the pre-lecture and come with questions. However, the pre-lecture does not go through very many examples and therefore leaves me with little understanding of how to stretch and apply the methods that they use to other problems. Basically, I don't know what I don't know until I start my HW.

2) We are often way ahead on HW and pre-lectures vs. the labs/quizzes and few practice problems that we do in class. So, by the time we get around to discussing it the HW is usually due that day or the next.

3) I struggled in my Classical Mechanics course with Newton's Second Law. My FBD's are usually spot on, but I have a little trouble translating those into second law equations and I struggle even more with manipulating the equations (say I have 3 unknowns & 3 equations.... I often go in circles and end up with a = a or a =1). Now, I am in the beginning of Electricity and Magnetism, but we are covering rotational mechanics first.

I love physics, I did great in the introductory physics course for which I had a different text and instructor. I just cannot seem to grasp this style of teaching/learning. I take great notes from the pre-lectures (I spend 1-1.5 hrs on the 15 minute video). I would take great notes in class as well if there was anything to take notes on.

I would love any pointers from people who had similar struggles or websites/books that would be good for someone in my situation.

extra context: I wouldn't say it is the math part that I have a hard time with. I am currently taking multivariate calculus and I am not struggling with that (nor have I with any of my other math classes).

2. Oct 11, 2014

### Staff: Mentor

iHere are Newton's Laws 1, 2, 3 in a nutshell
http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html

Newton's 2nd Law - http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt2cn

Limitations of N2L - http://hyperphysics.phy-astr.gsu.edu/hbase/limn2.html#ln21

http://hyperphysics.phy-astr.gsu.edu/hbase/limn2.html#ln22

A lot of people find calculus relatively easy, but applying it to understanding physics may be difficult for some.

It might be easy and straightforward to solve for some df(t)/dt, for y'(t) + ay(t) = f(t) or $\int f(t) dt$, but knowing how to apply that when f(t) = m(t), p(t), v(t), F(t), E(t), or combinations thereof, can be daunting at first.

Some physical relationships can be described with first or second order ODEs, but often the physics of a system requires PDEs or coupled PDEs, and sometimes nonlinear PDEs or coupled PDEs, depending on the complexity.

Last edited: Oct 11, 2014
3. Oct 11, 2014

### TehCylons

Thank you so much for your reply and the examples. You are absolutely correct about applications being my issue. It is easy for me to think about what physical forces are being applied and by what, but wrapping my head around exactly how it affects the behavior of a system is another thing entirely. However, with what I want to do it is SO important. So, I really want to get better at it. :)

Again, I really appreciate you taking the time to reply and I will check out your suggestions!

Edit: I just skimmed through them quickly and they are great. That is going to help a lot. Thank you again!