# [Basic question] What is the 'physical' explanation of a division by a fraction?

1. Nov 30, 2012

### cdux

For example, dividing by 2, means we cut something in two.

But dividing by 0.5, can only be explained with multiplying something by 2.

So, is there a "physical" explanation of dividing by 0.5? Is it "I divide by an entity that internally multiplies' or something as so bizarre?

2. Nov 30, 2012

### Number Nine

A number divided by 0.5 is equal to however many time 0.5 goes into that number. There's nothing more to it than that, and there's no reason it should have any physical significance.

3. Nov 30, 2012

### cdux

That's a good answer with a 'physical meaning':) "I fill that number with 1/2s".

4. Nov 30, 2012

### Staff: Mentor

I have \$37. How many quarters does that represent?

In case you're not familiar with US money a quarter is one-fourth of a dollar.

My question could be represented as 37/(1/4)

5. Nov 30, 2012

### cdux

That too. It's impressive how many ways there are to represent that concept in a physical form.

6. Nov 30, 2012

### symbolipoint

Division has another interpretation, simpler than "cut into" so many "equal pieces". Division means repeated subtraction of a quantity until no more of that quantity can be subtracted. Work with that idea and see what you find!

7. Dec 22, 2012

### Jeff Cook

"dividing by 2, means we cut something in two."

Not necessarily...the deeper meaning of a fraction to me is how many halves does it take to add up to a whole, as wholes are created before they are destroyed.

J

8. Dec 22, 2012

### Jeff Cook

Or by the order of operations, the whole is a product first. Then it can be divided. How does one crack and egg before it is created?

9. Dec 22, 2012

### cdux

I don't mind other interpretations but can you deny that it's correct to say it cuts something in half? It obviously cuts something in half. It's a valid interpretation.

10. Dec 22, 2012

### Jeff Cook

It's an expression so it could express just about anything you want, the past, the present or the future or something else. But the origins of the concept of division must follow after multiplication...don't you think? Else one runs into paradoxes and maybe even 2+2=5 if you continue forward. Order of operations, gotta follow them always.

11. Dec 22, 2012

### Jeff Cook

Take 2 right triangles back to back to form an isosolese. How many triangles do you have? 3? The two rights and the isosolese, the sum of the two? Take one away from 3, how many do you have? 1? 3-1=1? No...cause the whole is equal to the sum of its parts. Gotta follow that else all math is useless.

12. Dec 23, 2012

### rcgldr

One physical example would be the amount of linear force applied by a tire with a given torque and radius. The formula is force = torque / radius. If the same torque was applied to a tire with 1/2 the radius, the linear force applied by the tire would be doubled.

13. Dec 23, 2012

### HallsofIvy

You need to understand that there exist an infinite number of physical applications for any mathematical operation. There is no such thing a single "physical explanation".

14. Dec 23, 2012

### symbolipoint

When you divide by a number, that number by which you divide is a DENOMINATOR. You may often stop focusing on a physical meaning and at least temporarily, focus on the mathematical meaning.

Example: 10 divided by 0.5,
Meaning, $quotient =\frac{10}{0.5}$.
If you work the number properties correctly, you will find it is equivalent to 20.

15. Jan 1, 2013

### Staff: Mentor

Huh? How does it follow from the order of operations that a whole is first a product?

16. Jan 2, 2013

### HallsofIvy

Jeff Cook may be thinking that by "PEMDAS" multiplication comes before division. He is wrong of course. In fact, the order of multiplication and division is arbitrary- it just happens that "M" is ahead of "D". "PEDMAS", which is, in fact, used by some, would work as well. And even so, the order of operations is purely customary, having no relation to the "meaning" of any operation.

We could as well ask "How does one put things together before they have been separated?".

Last edited by a moderator: Jan 2, 2013
17. Jan 2, 2013

### Jeff Cook

The American Physical Society states that multiplication is of higher precedence than division with a slash, as do texts by Feynman, Landou, Lifgarbagez and others. The original question asked about the "physical meaning", and the physical world does require precedence of multiplication in a number of physical cases--not arbitrary at all. Problems arise in terms of vectors, in that if a function takes a scalar into values of imaginaries, one must deduce that it was already a vector on the complex plane (as i doesn't come out of thin air...it carries physical meaning that i was multiplied by x ahead of time). By not considering that x was already multiplied by i, paradoxes do arise (Grandfather Paradox is one that seems fitting for this discussion: the grandfather must have produced his son, for his grandfather to return back in time to kill him...such situations can only be resolved by understanding that the product arises ahead of the quotient even in cases of time reversal...if those may be so). In the same, one cannot equally ask, "how does one put things together before they have been separated?" because we only define a whole as equal to the sum of its parts. If it has no parts, then it is not a whole in the first place...and you cannot separate the whole if it doesn't exist, which is why you cannot divide by zero. There is no meaningful solution in doing so.

18. Jan 2, 2013

### Staff: Mentor

Please cite a specific publication by this organization.
Please cite a specific publication by any of these authors.
This doesn't make sense to me. If the domain of a function is real numbers, and the range is complex numbers, just because a complex number is produced doesn't mean that the input had to also be complex.
This is really a stretch, giving a situation that has never occurred as an example that is somehow related to your thesis.
This also makes no sense to me. Things do not need to consist of parts to be considered to exist. The early Greeks theorized that things were made up of atoms, which were indivisible. We know now that atoms consist of subparticles: protons, neutrons, and electrons. AFAIK, and I am not a physicist, electrons are indivisible -- they are "whole."
That isn't why. The reason is that division and multiplication are inverse operations. If a/b = r, then it must also be true that b*r = a. If b happens to be zero, and a is nonzero, there is no number r such 0 * r = a.

19. Jan 2, 2013

### 1MileCrash

I never understood where the idea of multiplication preceding division came from. Division is exactly multiplication by multiplicative inverse of the number we are "dividing by."

To claim that multiplication precedes division is to claim that certain multiplication precedes other types of multiplication which is nonsense because multiplication is commutative, it could never have any effect on the problem.

20. Jan 2, 2013

### Jeff Cook

@ CDUX,

Your post was straight forward and clear, and as easy to understand as "what is the physical meaning of subtraction?" If 4 parts amount to a whole, if you take away 2 parts, you get still 2 parts. If you take away 1/2, you get 3 1/2. Similarly goes for division in my answer to your reasonable question: the numbers in the denominator are the number of parts that add up to the whole. If you divide 4 by 2, then you have 2 parts in the denominator that still add up to the remaining whole. Same goes in the end for rational numbers, real numbers complex numbers or any other kind of number. Whenever you look at a fraction now, you can say with certainty what the denominator means and never run into any contradictions otherwise: it is the number of parts that add up to the whole. Failing to do so however, may (in certain cases) amount to paradoxes if you continue to take the problem farther. There really is no serious debate otherwise about such matters; they are backed by axioms back to Euclid's Elements and farther. All these matters can be found in standard mathematical literature, but I do hope you will continue to post questions as such to this forum and hope you will be able to avoid confusion in the process. My best.

Jeff

21. Jan 2, 2013

### micromass

I'll ask again: please cite specifically where they say that. If you can't, then please retract this statement. Otherwise, this is considered misinformation.

22. Jan 2, 2013

### pwsnafu

Except we do have mathematics where division by zero is allowed. See wheel theory.

23. Jan 3, 2013

### 1MileCrash

...

Is that not an entirely different algebraic system? How does that apply to what we are talking about (standard algebra)?

24. Jan 3, 2013

### micromass

I think the point is that standard algebra can be extended to a wheel (by adjoining some elements). So division by 0 can be defined by adding elements to the real numbers. Of course, in standard algebra (= real numbers), division by 0 is not defined.

25. Jan 3, 2013

### pwsnafu

Jeff makes statements like this
I keep getting the impression from Jeff's comments that he thinks that his opinions are universal truths. Wheels are an explicit counter-example, and as micromass points out, every commutative ring (includes integers, rational, reals and complexes) has an extension to a wheel, so if you wanted to do division with zero with the integers, you can enlargen your space to accommodate it.

I personally don't have much of a problem with Jeff's comments if we stick with Q (as per the thread title), my beef is that, well, he doesn't.

Last edited: Jan 3, 2013