Dimensional analysis of the equation u=prg

[Jimmy23]

I think you mean a negative times a negative is a positive.

Hi,

I know this is quite a basic question for you guys but it's helping me understand.

-(-b)-3b+c=1 or -a-(3a)+c=1. Are these both acceptable ways of doing the working out?

Also when you say 'I think you mean a negative times a negative is a positive'

Am I right in thinking the following is the process behind it:-
-a-(3b)+c=1 plug in b=-a
Then we get -a+(-3×(-a))+c=1 is that correct?

 

[haruspex]

-(-b)-3b+c=1 or -a-(3a)+c=1. Are these both acceptable ways of doing the working out?

They're acceptable equations, but I don’t see any working out there.

-a-(3b)+c=1 plug in b=-a
Then we get -a+(-3×(-a))+c=1 is that correct?

Yes.
I have a feeling from earlier posts that you are unsure of the basics of manipulation of equations. The fundamental rule is that if you have an equation (left hand expression)=(right hand expression) then you can apply whatever operation you like to both sides and still have a valid equation.
Thus if a=-b then you can multiply both sides by -1 to get (-1)a=(-1)(-b). Since minus times minus produces plus: -a=+b.
Or you can add b to both sides: a+b=-b+b=0.
But you have to be careful with division: from (a-b)(a+b)+(a-b)(a-b) you might want to divide both sides by a-b. That is ok as long as a-b is not zero; division by zero is illegal. So you would have to write "either a=b or a-b=a+b".

 

[Frabjous]

Most physicists will have have taken math courses at the upperclassmen level of college. Many will have specialized knowledge at the graduate school level.

Physicsforums is set up to help people with problems or specific topics. It can be a tool to help you improve your math skills, but it is probably not the best place to learn math.

If you are serious about improving your math skills, I would suggest looking at
https://www.khanacademy.org/
https://openstax.org/subjects/math
There are a ton of other sites. It’s just a question of finding one that you like (and you can always use physicsforums for your specific questions).

 

[jedishrfu]

Many of us have Masters degrees in various STEM fields. A few have PhD level in Math, Physics, Engineering and other fields. We work together as volunteers to maintain the site and help students with problems and conceptual understanding as well as discuss current science events and journal papers.

 

[Orodruin]

A few have PhD level in Math, Physics, Engineering and other fields.

... and indeed also a number of people that teach at university level in those fields (or taught if retired ...)

 

[lee1991]

Hi, I was wondering if anybody could advise me on where I’m going wrong or how I can move forward. I am doing an engineering qualification and struggling with my maths and will struggle to explain things properly. Like most, I have reached this stage and come stuck when it comes to solving the equation to get the value of a, b and c. Here’s what I have …

• L: 1 = -a -3b +c
• T: -1 = -2a -2c
• M: 0 = a+b

If 0 = a+b , then a = -b OR b = -a

I then tried using a = -b for the following equation and became stuck…

• L: 1 = -a -3b +c
• L: 1 = - (-b) -3b + c
• L: 1 = +b – 3b + c
• L: 1 = -2b + c
• L: 1+2b = c

 

[Orodruin]

then a = -b OR b = -a

No, both are valid. There is no “or” here. If a=-b then b=-a.

I then tried using a = -b for the following equation and became stuck…

• L: 1 = -a -3b +c
• L: 1 = - (-b) -3b + c
• L: 1 = +b – 3b + c
• L: 1 = -2b + c
• L: 1+2b = c

This does not seem like being stuck.

 

[lee1991]

No, both are valid. There is no “or” here. If a=-b then b=-a.

This does not seem like being stuck.

Thank you very much for your response. Yes, using OR was my poor choice of wording and thank you for correcting this. I'm stuck because I just don't know what to do after I reach the point 1 + 2b = c, in order to continue to work out the value of b, c and eventually a.

 

[Orodruin]

Thank you very much for your response. Yes, using OR was my poor choice of wording and thank you for correcting this. I'm stuck because I just don't know what to do after I reach the point 1 + 2b = c, in order to continue to work out the value of b, c and eventually a.

Well, you have only used 2 out of 3 equations. 2 equations can never determine 3 variables.