# Dimensional analysis

this question is about dimensional analysis involving a number with units and a number with no units, if the question is already answered in another post please redirect me if not here is a simple example, for example, :

say i have 2[in]-1. the 1 is dimensionless and the 2 has units of [in]. can I subtract the
2[in]-1=1[in]?or do i leave it as 2[in]-1.

this is not from a problem. i was doing dim. analysis on a sol. and ended up with this so i knew i did something wrong, but it got me wondering what to do in a problem that might be stated like this say in a multiple choice.

thanks for the time.

SteamKing
Staff Emeritus
Homework Helper
this question is about dimensional analysis involving a number with units and a number with no units, if the question is already answered in another post please redirect me if not here is a simple example, for example, :

say i have 2[in]-1. the 1 is dimensionless and the 2 has units of [in]. can I subtract the
2[in]-1=1[in]?or do i leave it as 2[in]-1.

Uh, no, you can't just promote pure numbers to dimensions having units on a whim. I'm not sure what 2 in. - 1 (pure number) even means.
this is not from a problem. i was doing dim. analysis on a sol. and ended up with this so i knew i did something wrong, but it got me wondering what to do in a problem that might be stated like this say in a multiple choice.

thanks for the time.

You can add and subtract only like units. Dimensionless numbers have no units, thus they cannot be added or subtracted to any quantities which contain units.

• nosequeponer
Uh, no, you can't just promote pure numbers to dimensions having units on a whim. I'm not sure what 2 in. - 1 (pure number) even means.

You can add and subtract only like units. Dimensionless numbers have no units, thus they cannot be added or subtracted to any quantities which contain units.
ok thanks for taking the time and clarifying.