# Physical units and dimensions

• Taturana
In summary, the conversation discusses the concept of dimensional analysis in physics and how it relates to physical units. The participants also explore the meaning of "dimension" in different contexts and how to interpret complicated units. They suggest varying parameters in equations to better understand their effects.

#### Taturana

Hi =D

I was thinking about the physical units (g, m, s, K, J, A, V, etc). I know we got a thing in physics called dimensional analysis, so you analyse some variable that you know it depends on other variables and the two sides of the "equation" (I mean a equation with the proportional sign instead of equal sign) have to have the same "dimension" (the same units).

We also use to talk: "The dimension of velocity is the dimension of space over the dimension of time.".

Why do we call "dimension" the units of physics? Has this term "dimension" the same meaning of when we talk that time is the fourth dimension in space-time, of when we talk that a equation like "x + y + z = 0" describes a plane?

What's the correct way to imagine the dimensions of the physical units in a graph? What about a complicated concept that has lots of base units like electric current (measured in A), or even a simple one (Joule = kg * m^2 / s^2)? How should I imagine this complicated combination of dimensions? Does it has really intuitive and physical meaning, or is a pure result of the mathematics?

Other questions...

I know how to understand a unit like Watt: Watt is equals to Joules per second, so if we have a resistor that dissipates 2W of power we can talk that this resistor converts 2 Joules of electrical energy into thermal energy (heat) in one second.
But what about units like N*s (unit of impulse), or Joules (N*m)? How do I understand that? I have a understanding of these units but I don't know if it's right so check it please: I understand that if we have 5J or 5N*m I have two values encapsulated in this number: we of course don't know what are the values separately but we know that they multiplied is 5, and I can also conclude that we have two physical concepts encapsulated on unit Joule (force in N and space in m), that's right? Am I thinking right?

Any contributions are welcome... Thanks

Taturana said:
Hi =D
But what about units like N*s (unit of impulse), or Joules (N*m)? How do I understand that? I have a understanding of these units but I don't know if it's right so check it please: I understand that if we have 5J or 5N*m I have two values encapsulated in this number: we of course don't know what are the values separately but we know that they multiplied is 5, and I can also conclude that we have two physical concepts encapsulated on unit Joule (force in N and space in m), that's right? Am I thinking right?

Any contributions are welcome... Thanks

As for this question, think of the unit like "man-hours". If a job requires 100 man-hours to finish, it could be accomplished in 10 hours with 10 people, or in 4 hours with 25 people, etc. you are right that we don't know what the values are separately. We don't know that a job that requires 100 man-hours to finish requires 1 or 10 or 25 people. We just know it requires a combination of workers * hours = 100.

I actually has the same question with yours initially, thinking in terms of dimensions really helps understanding the physics, however, when equation get more complicated, dimension analysis becomes less efficiency for understanding the physics. So I changed my habit to vary the parameters in the equation and see the effect, eg, v=L/t, if t is shorter, then v is larger. This kind of thinking is quite useful when dealing with complicated equation. I think:)

## What are physical units and dimensions?

Physical units are standardized measurements used to quantify physical quantities, such as length, mass, and time. Dimensions refer to the fundamental properties of a physical quantity, such as length being a one-dimensional quantity.

## Why are physical units and dimensions important in science?

Physical units and dimensions allow scientists to accurately measure and compare physical quantities. They also help in the development of theories and equations that describe natural phenomena.

## What is the difference between a base unit and a derived unit?

A base unit is a fundamental unit of measurement, such as meters for length, while a derived unit is a combination of base units, such as meters per second for speed.

## How are physical units and dimensions related to the SI system?

The International System of Units (SI) is the globally recognized system of measurement that is based on seven base units, from which all other units are derived. It also establishes the rules for expressing units and performing unit conversions.

## What are some common prefixes used in the SI system?

Some common prefixes used in the SI system include kilo (k), mega (M), giga (G), milli (m), micro (µ), and nano (n). These prefixes indicate powers of 10 and are used to express larger or smaller quantities.