# I need a lot of help with Kinematics and Dynamics so forces and motion

1. Oct 15, 2012

### ZeshShawn

I need a lot of help with Kinematics and Dynamics so forces and motion would be what you can call that.

I have a teacher that does not teach, I have had her for two years in a row now so if I had some knowledge from grade 11, It would still help me but I learned close to nothing in Grade 11 and same goes for grade 12.

But now, this is Senior year and I want to go into medicine so I really need to do good in her class, I'm trying my best but It's just too complicated without anyone guiding me even a little bit. You ask her a question, instead of actually teaching you how to do it, she'll just end up giving you the answer. My other two classes, I'm doing very good in because the teachers are helpful and can actually teach, they're dedicated and what not. This teacher just gives out a lot of work and does not teach at all. I'm stuck in Physics and I can't afford to get lower than a A in this class.

Can someone quickly run-through how I can overcome with forces and motion and the basic fundamentals I need to know as a unit-test is coming up. I know it requires thinking and such but I have issues with "re-arranging" the formulas and such so any help will be appreciated.

I am also doing Circular Motion soon so.. any help on that would be appreciated as well.

Thanks a lot.

2. Oct 16, 2012

### Angry Citizen

Re: Kinematics/Dynamics

Always identify the variables you are asked to find before you ever write an equation.

Translate all the words in the problem into mathematical statements. If a vehicle starts from rest, v0 must equal zero. If a body doesn't accelerate, the sum of the forces must equal zero. If a body is moving in a vacuum or on a "smooth surface", or if drag/friction is not mentioned in the problem, conservation of energy applies, and if so, it will make your life easier.

You need x equations for x unknowns. Learn it, live it. Take it as gospel.

Fall in love with motion along a single line. The rotational analogs are pretty much exact copies of that motion, and once you learn to think of it like that, they're a breeze.

If you're in calculus-based physics, learn how to derive the formulas. It's not hard.

Don't put a single number on the page until you already have the answer. Write everything in letters, then once it's nice and neat, plug in numbers. This is also helpful for the next piece of advice:

Check units check units check units. If you're asked for a force, there better be kg-m/s^2 (or the English equivalent) in your answer.

If you don't remember the formula, you can almost always find it by thinking about units. If you knew nothing about physics, you could still figure out that F=ma is valid because forces must be kg-m/s^2.

Vectors are the most useful thing you have ever seen, and yes, they even surpass air conditioning. Learn vector algebra really well, it's not hard. X usually involves cosines, y usually involves sins. When you're dealing with normal forces, sometimes it's helpful just to reverse that mantra.

That's all I got.

3. Oct 16, 2012

### Staff: Mentor

Re: Kinematics/Dynamics

Nice post, oh Angry one!

4. Oct 16, 2012

### Angry Citizen

Re: Kinematics/Dynamics

Thank you!

5. Oct 16, 2012

### jhae2.718

Re: Kinematics/Dynamics

THIS

To OP:

Analytical dynamics is infinitely easier and infinitely more insightful when using a vector (or tensor) formulation. From your post, I am presuming you are doing mechanics at the high school level. Unfortunately, such treatments are often scalar formulations.

Ultimately, the dynamicist is interested in deriving the governing equations of motion. These are second-order ordinary differential equations that can be solved to give the position of the object of interest.

The professor I learned my philosophy of dynamics from had a six step formulation when using Newton's laws. (There are other methods to find the EOM, such as Lagrange's equations shown in my .sig, but you'll learn those much later on.)

1. Identify the system
2. Draw a free-body diagram
3. Establish relevant reference frames
4. Write a vector representation of the forces
5. Perform vector kinematics up to the velocity level
6. Use Newton's second law to write the governing equations of motion

These steps will work for a point mass. Rigid bodies require extra work.

Angry Citizen's post will probably be more useful to you in the short term, but I wanted to express the "proper" way of solving problems.

If you are taking calculus based physics, I did make a blog post that derives some of the basic kinematics for a Cartesian plane. https://www.physicsforums.com/blog.php?b=2945 [Broken] (For rotating reference frames, you'll need the kinematic transport theorem, one of the most useful things in dynamics.)

I will probably write a tutorial on rigorous analytical dynamics, at least in the planar case, at some point...

Last edited by a moderator: May 6, 2017
6. Oct 16, 2012

### jhae2.718

Re: Kinematics/Dynamics

Also, do lots and lots of problems. When you're done with those, do more.

I'm not sure what your mathematical background is, but if you can I would recommend learning how to derive expressions. This is a personal thing; I have a rule that "If I can't derive it, I don't understand it."