# A question about units

1. Jun 16, 2015

### Deepak K Kapur

Aa...ha.... I can log in now....
So, without wasting much time (one never knows how much time one has...)
My next question....

I saw the list of derived units on wikipedia. All of them had the basic units either multiplied or divided.

Why aren't basic units added or substracted to get derived units?

2. Jun 16, 2015

### rumborak

Well, you can't subtract different units from each other. I mean, what would "1 kg - 1m" mean physically?
In they end, units are dimensions. You can multiply dimensions (e.g. volume of a box is X*Y*Z), but you can't add/subtract them.

3. Jun 16, 2015

$175 - 327 hippopotamuses = ?? 4. Jun 17, 2015 ### Deepak K Kapur Ha...ha Well,$175 - 327 hippopotamuses = $75 ( because 327 hippos cost$100)
I mean to say a relation can be found in addition or substraction also...

5. Jun 17, 2015

That is $175 - 327 hippo * (100/327$/hippo) = $175 -$100 = $75 You only add or subtract same units. Never different units. Ever. 6. Jun 17, 2015 ### Deepak K Kapur So,$-hippo or $+ hippo is not allowed but...$ x hippo is allowed (if such were the case in reality)
Why so...multiplication is repeated addition only...

7. Jun 17, 2015

### A.T.

What application would that have?

8. Jun 17, 2015

### Staff: Mentor

Multiplication of integers can be represented as repeated addition. Not multiplication of reals.

9. Jun 17, 2015

### Deepak K Kapur

What is the limitation in doing so with reals?

Now...

10 dolls + 15 balls = 25 toys ( a new unit, I suppose)
and...
10 dolls x 15 balls = 150 toys

So we have distributed 'reals' in the second case..

10. Jun 17, 2015

### A.T.

These aren't physical units. In terms of physics all those counts are dimensionless numbers.

11. Jun 17, 2015

### Staff: Mentor

Your toys aren't a valid unit because each unit is not the same. A meter is a meter, and two different meters are not distinguishable. The same is not true of your toys unit. So it isn't a valid unit.

Your question has been answered. This forum is for education and not for debate.

If you wish to further push your idea of adding dissimilar units then you must provide a professional scientific reference supporting the practice. Failure to do so is continued personal speculation after being taught the correct physics.

12. Jun 17, 2015

### Deepak K Kapur

Ok. I got your point.

I DARE NOT post more....

13. Jun 17, 2015

### Staff: Mentor

To extend that example:
15 dolls + 10 balls = 25 toys as well.
Therefore,
10 dolls + 15 balls = 15 dolls + 10 balls
Which reduces to
5 balls = 5 dolls
Divide by 5:
1 ball(s) = 1 doll(s)
you can also derive that every toy is a ball:
1 toy(s) = 1 ball(s)
which is certainly not what the calculation was supposed to mean. You can assume it to be true in terms of mathematics, but then there is no point in giving the same unit (toys) three different names.

14. Jun 17, 2015

### Staff: Mentor

To coin a phrase, "you can't add apples and oranges."

Chet

15. Jun 17, 2015

### Staff: Mentor

Quick add:

Math isn't really debateable and not necessarily even explainable in this way. It is a human invention, agreed upon by convention, and has the form it does, simply because it works. It doesn't need any other reason.

It is similar to other languages in this regard. There may be certain things about the English language you don't like, but you aren't entitled to change them unless you rise to a position of being a prominent expert who can drive the discussion to change the consensus (a discussion we won't entertain here).

So all this attempt to argue with the reality of how math works is pointless.

16. Jun 17, 2015

### Deepak K Kapur

Having gained strength, I say.....

Does it mean, then, that math is just a brute fact...

In other words, would it suffice to say that....
THIS IS THE WAY UNIVERSE WORKS and THAT'S ALL!

17. Jun 17, 2015

### rumborak

I disagree with that it's just an idiosyncrasy of math.
As I said before, units should be viewed as "markers" for a dimension. That dimension can be meter, or it can be "toy". So, while it may look tempting to consider a multiplication of units as an adding of them, that part really only applies to the scalars (e.g. the 5 in 5m) that we attach to those dimensions. The dimensions stand unchanged. That is, while a "5*2" gets mapped back to a single scalar, the "kg*m" stays a "kg*m".

18. Jun 17, 2015

### Staff: Mentor

I would say that the derived units come out of the equations that compute those quantities:

area = length x width if length and width are in meters and since area = length x width then the units of area are meters x meters or m^2.

speed = distance / time so if distance is in meters and time in seconds then units for speed are meters/seconds.

19. Jun 17, 2015

### Staff: Mentor

No, this isn't about how the universe works, it is just how math works. There is no implication that any particular equation reflects how the universe works and the structure of math doesn't necessarily say anything at all about the universe.

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