How about…
You have a big box of apples. You eat two a day.
Q:How many more apples will you have in 3 days?
A: (-2) * (+3) = -6 more
Q: How many more apples did you have 4 days ago?
A: (-2) * (-4) = +8 more
I don’t know if this will work for a 12 year old. In electronics, power is calculated as the product of curerent and voltage and is an absloute value. Thus:
12 volts × 2 amps = 24 watts.
-12 volts × -2 amps = 24 watts.
When voltage is labeled as being negative, it simply means the polarity is reversed. Negative current simply means current flowing in a direction which is opposite a given reference point. Whether the voltage and current are negative or positive makes no difference to a simple load like a light bulb; it will glow just as brightly and get just as warm. Therefore, the power consumed is identical no matter what the signs of the voltage and current are.
Because you put a comma in.
“Don’t you pay no nevermind.”
[hijack]
Except that isn’t right. Or, at best, it’s only correct in some dialects of English, none of which have really caught on.
Historically, the double negative reinforces the negative. For example, Chaucer said in his Canterbury Tales “Ther nas no man nowher so vertuous,” meaning there was no man so virtuous anywhere. Shakespeare, in Twelfth Night, had Viola say of her heart “Nor never none/Shall mistress of it be, save I alone,” meaning no one except herself would ever be mistress of her heart.
More recently, Al Jolson said “You ain’t heard nothin’ yet,” meaning the audience hadn’t heard anything yet (The Jazz Singer). President Reagan, in his time in office, used the construction “You ain’t seen nothin’ yet,” with an analogous meaning.
Clearly, the traditional prohibition against double negatives don’t mean nothin’.
“The American Heritage Book of English Usage”
[/hijack]
This is a fantastic real life example, just what the kids need to see. This is the kind of stuff that makes them want to learn math for a change. Thanks!
I think by reading the thread everyone sees why I didn’t want to become an English teacher. Too many freakin’ arguments. Math is always true, pure and simple.
I don’t think that’s what the OP is looking for. This is not an example of multiple of negative numbers being positive, it’s an example of a situation where the sign is irrelevant. (+12 v * -2 A) is also +24 watts, as is (-12 V * +2 Amp).
When you show me how to switch the voltage source (-12V) and keep current going in the same direction (2A), scr4, you might genuinely be on to something. 
I agree that explicit double negatives are usually understood as reinforced negatives, but I’d say there’s plenty of examples of double negatives that are commonly interpreted as alleviated positives.
First of all, look at examples like “not bad.” It’s a question of semantics, but I think one can fairly say that the best way to define “bad” is the negative of “good” (while I wouldn’t define “good” as “not bad”: “bad” derives its meaning from good, but “good” has a meaning in its own right). Clearly, most people would understand “not bad” as an alleviated form of “good”: Not really good, but OK.
In the good/bad example, this doesn’t work if you deny that “bad” is simply the negative of “good,” nothing else. Now look at, say, “unimportant,” which clearly is the negative of “important” and derives all of its meanig from that word. “Not unimportant” would surely be understood as “important,” not as “totally unimportant” as it would if a double negative were a reinforced positive. Sort of a litotes.
Take Orwell’s Newspeak: What would you think “not ungood” would mean? I bet most people would say it’s “not bad,” i.e. an alleviation of “good.”
Or take this: I’m trying to say that there has never been a famine in country X. I could say: “There has never been a time when there was no food in X,” or I could shorten it down to: “Never has there been no food in X.” This last sentence surely isn’t grammatically perfect, and it’s a bad example because I couldn’t think of a better one right now, but I think people would base their interpretation of it on the context in which I say that.
What I’m trying to say is: Double negatives are sometimes interpreted as positives, sometimes as reinforced negatives by people, depending on what they think the speaker wants to say in that context.
The rhetorical term for this is litotes (lie-TOE-tees), i.e. negating the opposite of what you intend. “She’s not unattractive.”
Helium-filled balloons. Each has a lift of 2 ounces, i.e., a weight of -2 ounces. You’re standing on a scale, and someone gives you 3 of them, so the change of your weight is 3*-2 = -6 ounces, or a decrease of 6 ounces. You let them go, the change of your weight is -3*-2 = 6 ounces, or an increase of 6 ounces.
More explanations (including some already given) are at http://mathforum.org/dr.math/faq/faq.negxneg.html
Look all you want, but that’s not what is meant by “double negative.” Take what Quirk and Greenbaum say: “In substandard English, there is an entirely different form of multiple negation where more than one negative form is used, but the meaning is that of a single negative: No one never said nothing.” (Italics mine). The discussion above this note says “If a clause contains a negative element, it is usually negative from that point onward.” So standard English would be “No one ever said anything.” But “No one never said nothing” says exactly the same thing. It does not mean everyonw was talking, and no one would construe that way.
What you’re describing is a particularly literary form (I know there’s a name for it, but I’m drawing a blank on it) that does indeed imply a negation. It is not, however, a double negative as the term is commonly used.
That’s not the most stunning example, RealityChuck, as there are three negatives resulting in a negative–which is exactly what we’d expect.
Perhaps this does work well for this age group, but I have to say that’s a pretty confusing method of multiplication. It give the distinct impression that multiplying with negative numbers is somehow not commutative. In some cases you end up facing the opposite direction even though the mathematical result is the same.
You are wrong! Suppose an electric device with 2 terminals A and B and let’s label A the positive terminal.
A positive voltage means that the voltage at A is greater than at B. A positive current means that the current enters the device at A and leaves at B.
If you take an electric bulb and apply +12 V to its terminals (Va > Vb), a current of + 2A will flow from A to B and the bulb will dissipate +24W.
If you reverse the polarity (Va < Vb) a current will flow from B to A (-2A according with our reference) and the bulb will dissipate (-12V)x(-2A) = +24W.
In a battery, the current flows from the terminal labeled - to the one labeled +. So, you would have (+12V)x(-2A) = -24W, meaning that the device is delivering power, instead of dissipating it.
I don’t have apples.
means about the same as
I have no apples.
I don’t have no apples.
technically means about the same as
I have apples.
because the two negatives ought to be multiplied.
But speakers who incorrectly (or “non-standardly”) use double negatives are usually aiming for reinforcement:
I don’t have no apples.
is intended to mean the same as
I don’t have apples.
because the speaker wants the negatives to be added together.
No. Wrong. That simply isn’t correct.
Grammar is not logic.
Grammar is not math.
Grammar is not algebra.
Grammar is grammar, and it isn’t bound by any law other than usage.
At one time, people thought English grammar should conform to Latin grammar. From this moronic premise, they derived a host of idiotic laws, one of which is the prohibition against splitting an infinitive. What, you may ask, is splitting an infinitive? In the sentence “To boldly go where no man has gone before.”, the word `boldly’ is splitting an infinitive. That sentence is perfectly comprehensible and, therefore, well-formed according to all rational laws of English grammar, and Latin grammar can go hang. Yet for generations, grammarians made themselves look like jackasses trying to enforce a nonsensical rule.
The moral? Don’t try to shoehorn grammar into something it isn’t.
[hijack]
Why was splitting an infinitve such a `bad idea’ according to those grammarians long dead? Because in Latin, the infinitive is all one word. Therefore, it’s impossible to split an infinitive in Latin.
[/hijack]
You are right. Grammar consists of a set of arbitrary rules, specific of a given language, that have nothing to do with math or logic.
In Portuguese you say “Eu não tenho nenhuma maçã” (literally “I don’t have no apples”). The same is true in Spanish and Italian. In French the default is the double negative: “ne…pas”, “ne…rien”.
Then it’s a question of definition. I’m not denying that everybody would understand “No one never said nothing” as “No one ever said anything,” as standard English would put it (although, as erislover has already pointed out, this is a triple negative). Of course if you define “double negative” so as to mean those substandard constructions only, excluding “she’s not unattractive” and the like, then a double negative indeed is a negative, not a positive.
Particularly interesting is a quote from the theme song of the TV show “Family Ties”: “And there ain’t no nothing we can’t love each other through”. Here, we have a quadruple negative, with positive intended meaning (“We can love each other through anything”). The first three negatives (“ain’t no nothing”) are meant to reinforce each other to produce an overall negative, while the fourth negative (“can’t”) reverses that negative meaning to a positive.
I disagree that in French, the double negative is the norm, since a sentence with “ne” but without “pas”, or vice-versa, would be ungrammatical. The “ne… pas” construction altogether is one single negative. If we’re insisting that grammatical negatives are multiplicative, then I suppose that would make “ne” or “pas” by itself imaginary.
Incidentally (but this is getting a bit afield), I’ve never understood the controversy over “split infinitives”, since it’s not possible to split an infinitive in English, either. “To boldly go” is not a split of “to go”, the infinitive form of the verb “go”; it’s the infinitive of the verb “boldly go”, composed of the adjective “boldly” and the primitive verb “go”.