Dimensional analysis of the equation u=prg

[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.