# Understanding the Equations and Units

• B
I have been struggling through my physics class this summer (it doesn't help that 5 months worth of material is crammed into 2 months in an online classroom). This site has helped me out a lot, but I am still stumbling through understanding when to use certain equations. When I started reading the book, it would give an equation, and then suddenly it would say to use another equation to solve a problem because you plug the first equation into a second equation to get a third equation. It baffled me. If I understood when to use certain equations, I think I would do a lot better. Is there a simplified chart or tutorial that can help me understand all of these equations better?
On a side note, I don't understand when to use m/s vs. m/s^2. Any tips?

SteamKing
Staff Emeritus
Homework Helper
I have been struggling through my physics class this summer (it doesn't help that 5 months worth of material is crammed into 2 months in an online classroom). This site has helped me out a lot, but I am still stumbling through understanding when to use certain equations. When I started reading the book, it would give an equation, and then suddenly it would say to use another equation to solve a problem because you plug the first equation into a second equation to get a third equation. It baffled me. If I understood when to use certain equations, I think I would do a lot better. Is there a simplified chart or tutorial that can help me understand all of these equations better?

The key to doing well in physics is not to memorize which formula to use to solve which problem, but to understand what the formula is telling you about a certain physical situation.

On a side note, I don't understand when to use m/s vs. m/s^2. Any tips?
When you learn the name of a new physical quantity, for example, velocity or acceleration, learn the units which are associated with that quantity. After all, how you measure the quantity implies the use of a certain set of units.

Also a good rule to learn and use is substitution. In other words, when you need to use 2 equations, find a common unit and make one equation equal to the other. Then you have a new equation you can use for the specific problem. m/s = velocity; m/s2 = acceleration (but you knew that).

CWatters
Homework Helper
Gold Member
One reason to make a third equation from two others is to eliminate an unknown variable. Look up simultaneous equations.

sophiecentaur
Gold Member
On a side note, I don't understand when to use m/s vs. m/s^2. Any tips?
Those two units refer to different quantities - on is Velocity and the other is Acceleration so they are not interchangeable. That's something you should always watch out for.
The idea of choosing from a toolbox of equations and going through chains of calculations or using substitutions is a hard one to get used to. You will always look at a worked example and ask yourself "why did he do that?". Very frustrating, I know and the only way you can get over that is by doing lots of example questions yourself and learning all the relevant equations so well that you have them all at your fingertips. Things can only get better as you progress through the process but don't expect for it all to happen quickly. Sorry!
The set of Equations of motion under constant acceleration are a good example of where you have to know them so well that you can always pick the one with only the variables that you know - plus one unknown. I know a lot of students who want to use a particular equation and get cross when it doesn't do the job. haha. Also, the basic electrical equations R=V/I, P = VI etc can also be re-arranged and re-stated at a confusing rate to the beginner but, again, practice makes it automatic and it becomes second nature.

The key to doing well in physics is not to memorize which formula to use to solve which problem, but to understand what the formula is telling you about a certain physical situation.

When you learn the name of a new physical quantity, for example, velocity or acceleration, learn the units which are associated with that quantity. After all, how you measure the quantity implies the use of a certain set of units.
Thanks for thinking I'm so incompetent that I don't take notes. This class is online - it was the only option - and it is very hard for me. I'm only sorry you were the first one to reply. It shows me that this community isn't as good as I thought. Bad first impression. Oh well.

Those two units refer to different quantities - on is Velocity and the other is Acceleration so they are not interchangeable. That's something you should always watch out for.
The idea of choosing from a toolbox of equations and going through chains of calculations or using substitutions is a hard one to get used to. You will always look at a worked example and ask yourself "why did he do that?". Very frustrating, I know and the only way you can get over that is by doing lots of example questions yourself and learning all the relevant equations so well that you have them all at your fingertips. Things can only get better as you progress through the process but don't expect for it all to happen quickly. Sorry!
The set of Equations of motion under constant acceleration are a good example of where you have to know them so well that you can always pick the one with only the variables that you know - plus one unknown. I know a lot of students who want to use a particular equation and get cross when it doesn't do the job. haha. Also, the basic electrical equations R=V/I, P = VI etc can also be re-arranged and re-stated at a confusing rate to the beginner but, again, practice makes it automatic and it becomes second nature.
Ok, thank you. Yeah, that's what I have trouble with - deciding which equations to mash together in order to get the desired result and get the correct answer.

SteamKing
Staff Emeritus
Homework Helper
Thanks for thinking I'm so incompetent that I don't take notes. This class is online - it was the only option - and it is very hard for me. I'm only sorry you were the first one to reply. It shows me that this community isn't as good as I thought. Bad first impression. Oh well.
You can still take notes even from an online class.

If your academic career proceeds further, you're still going to need to take notes and then review what you've written down.

You won't learn effectively if others do the work for you.

sophiecentaur
Gold Member
Ok, thank you. Yeah, that's what I have trouble with - deciding which equations to mash together in order to get the desired result and get the correct answer.
Looking at each of the available equations and eliminating the ones that have variables in them that are unknown to you, will reduce the confusion. Write the equations down without looking at the book - learning by repetition. This is looked on as an out-dated method but all the successful people I know have done a similar thing in order to learn. (The Formula Sheet that is included in nearly all exams has been a bad influence on the ability to learn stuff). There is not time to look through that list if you want to be earning marks quickly enough in an exam.

You can still take notes even from an online class.

If your academic career proceeds further, you're still going to need to take notes and then review what you've written down.

You won't learn effectively if others do the work for you.
Again, you think I'm so incompetent that I don't take notes? You don't listen very well. I take very good notes. But writing things down doesn't mean one understands everything right away.

berkeman
Mentor
Again, you think I'm so incompetent that I don't take notes? You don't listen very well. I take very good notes. But writing things down doesn't mean one understands everything right away.

jedishrfu
Mentor

http://www.smarterthanthat.com/physics/physics-dont-panic-10-steps-to-solving-most-physics-problems/

The problem with your first course in physics is that you can't solve the interesting problems from first principles but must rely on someone providing you with a formula that works under a specific set of conditions. There is no cookbook that you can go to like you would with say leaf identification in biology where questions are asked about the leaf texture, size, shape... and from that you are led to identify the tree or plant.

However, you could begin to develop such a resource by reading over what you have to see what conditions were important to select the formula of choice. You could also categorize the problem types like whats done in a physics review book. You must start to think like Holmes and consider the very words of a problem asking yourself why did they provide that information.

Here's a Dummies resource too that categorizes problems and relevant equations:

http://www.dummies.com/how-to/content/physics-equations-and-formulas.html

You may also ask yourself how do physicists solve these problems and the answer is from first principles. Basically a physicist would re-derive the formulas that you are given using the basic facts of the problem and then simplifying things down to the very same formula that you have now (of course most physicists dont need to do this as it has already been lasered into their brains but I digress).

So don't panic and consider what advice you've been given and hang in there.

Also please don't get upset with SophieCentaur or SteamKing, they are part of an elite team of volunteers at PF and have helped many students who've listened to their advice and not in how they worded it. We can never know what a student knows unless they tell us and so we can't assume anything.

Take care,
Jedi

Evo and berkeman
jedishrfu
Mentor
Thank you all for your contributions, I think we've really helped the OP.

Since the thread has run its course, I am now closing it.

Thank you again,
Jedi