# Painful confession time: Things you \*ought\* to know but \*don't\* **URL:** \ **Category:** In My Humble Opinion\ **Created:** [April 30, 2010, 6:12am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836 "2010-04-30T06:12:48Z")\ **Posts on this page:** 20\ **Page:** 8
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 17, 2010, 2:35pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/141 "2010-05-17T14:35:01Z")
> [@](#): > > How on earth do you “multiply” a kilogram by a meter (fine, a meter squared), and how do you “divide” either one of them by a second squared (which itself doesn’t make much sense). You’re asking how you release that equivalent amount of energy, and thinking that you maybe have to have the pencil (or whatever) moving at the speed of light. But it doesn’t work like that. To accelerate a pencil to the speed of light would require an infinite amount of energy. You’d release the equivalent energy of that pencil if you annhilated it with anti-matter having the same mass. Annihilated it with an anti-pencil of the same size. as for the literal question you asked – people do such odd things with units all the time. How do you "divide’ a meter by one “second”. You do that all the time when you measure velocity. Sometimes these units combine in unexpected ways. ---
**Author:** ![bouv](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@bouv](https://boards.straightdope.com/u/bouv)\ **Post date:** [May 17, 2010, 3:14pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/142 "2010-05-17T15:14:14Z")
> [@Koxinga](#): > > How on earth do you “multiply” a kilogram by a meter (fine, a meter squared), and how do you “divide” either one of them by a second squared (which itself doesn’t make much sense). Don’t think of it as a meter divided by a second so much as a meter _per_ second, if that helps. Every second, you go one meter. If you’re still having trouble grasping the concept of multiplying and dividing units…well, I don’t know what to tell you. That’s how different physical qualities are related. Mass is kg, distance is m, time is s. A speed/velocity is meters per second, or m/s. A momentum is kilogram meters per seoond, or kg\*m/2. Maybe this will help: The speed you’re walking is meters per second, or m/s. The _acceleration_ is going to be an increase (or decrease) in that speed (well, technically velocity, but that part’s not important right now.) That aceleration is a _change in the speed_. This acceleration is measured in meters per second, _per second_, or m/s/s. This can also be written as meters per second _squared_ (if you don’t get why m/s/s is also m/s[sup]2[/sup], well…it’s a property of fractions, so go talk to a mathematician, :p) Now, this still might not make sense, but think of it this way: when you speed up, you are increasing your meters per second. How do you measure this increase? Well, you say that every second, you will note how much the speed increases. So in other words, every second, you measure the increase in meters per second, so it comes out to meters per second per second…that first meters per second is the speed, and the other second is the unit of time you are measung that increase in, so it’s (speed increase) per second. It can then also be written as m/s/s, or most commonly m/s[sup]2[/sup], which are the SI units of acceleration. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 18, 2010, 2:54am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/143 "2010-05-18T02:54:00Z")
> [@Koxinga](#): > > How on earth do you “multiply” a kilogram by a meter (fine, a meter squared), and how do you “divide” either one of them by a second squared (which itself doesn’t make much sense). I think what’s frustrating the nerds here is that you do this kind of thing yourself every day. I guess most people just don’t think about it explicitly. Examples: 1. miles/hour 2. dollars/doughnut 3. dollars/gallon 4. candies/dollar 5. kids/classroom 6. strings/guitar \* guitars \* dollars/string = dollars I need to spend on guitar strings. On the literal face of it, none of these make much sense (“HOW are you dividing candies by dollars?”), but it’s the only way to get practical answers. ---
**Author:** ![Yumblie](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@Yumblie](https://boards.straightdope.com/u/Yumblie)\ **Post date:** [May 18, 2010, 3:08am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/144 "2010-05-18T03:08:23Z")
Remember that a meter or a kilogram aren’t physical things, they’re just arbitrary measurements. You can multiply a kilogram by a meter because they are just abstract concepts, you can do whatever you want with them. You can’t multiply a pencil by _c_[sup]2[/sup], but you can multiply the pencil’s mass in kilograms by _c_[sup]2[/sup]. The mass is just a number, expressed in units of our choosing based on convenience. The mass can be 1 kilogram, 0.7 pounds, 155 hoozawhatsits, 99.7 hoobajoobs, whatever. Typically, kilograms are used because the common unit for energy is expressed in kilograms times (meters per second) squared. Why do these units together in that combination yield something we call energy? Well, it goes back to basic physics concepts you can find in any intro physics textbook, but this hijack has probably gone on far enough… ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 18, 2010, 3:09am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/145 "2010-05-18T03:09:59Z")
> [@bouv](#): > > Now, this still might not make sense, but think of it this way: when you speed up, you are increasing your meters per second. How do you measure this increase? Well, you say that every second, you will note how much the speed increases. So in other words, every second, you measure the increase in meters per second, so it comes out to meters per second per second…that first meters per second is the speed, and the other second is the unit of time you are measung that increase in, so its (speed increase) per second. Or to try my “practical explanation method” that I try in physics class: Your car will get up to 80 mph. Any car will. The difference is that a Camaro will do it in about 8 seconds, and your minivan will probably take 16. So the difference isn’t the speed, but the acceleration: the rate of change in speed. So, the change in speed from 0 to 80 is the same, but the _time_ taken will be different. If the time taken is smaller, then the acceleration is greater. So if I went from 0-80 in 8 seconds, my average acceleration is 10 miles/hour_second. If you take 16 seconds, yours is 5 miles/hour_second. My acceleration was faster than yours, but to show it, we needed to divide by the time it took to do it. When we divide by a smaller number (seconds) we get a bigger answer (acceleration). My answer is bigger than yours, in terms of acceleration. If both times were measured in seconds, then we have s\*s on the bottom of the fraction, which is s^2. Oh, I had another thought about your objection to dividing by another unit. Don’t think of it as dividing by this other unit, but as rating one in terms of another. You don’t usually think of dividing your dollars by a gallon of gas, but everyone rates a gallon of gas in dollars. The one that you want to emphasize is on the bottom of the fraction: the one that is a constant. The size of a gallon of gas doesn’t change, so I put the variable dollars on top. At least most people like it that way. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 18, 2010, 3:11am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/146 "2010-05-18T03:11:17Z")
Hell, if anyone has any clear idea of why the speed of light shows up in calculating energy, please tell me, because I have NO idea. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [May 18, 2010, 4:35am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/147 "2010-05-18T04:35:22Z")
> [@](#): > > It’s not saying that at all. The m and c2 are on the same side of the equals sign. They aren’t equal to each other, they (multiplied together) are equal to some amount of energy. If the equation were this: > > c2 = m > > then you would be right…but that’s not the equation. > > The energy (the big E on the left of the equation) is what is being measured and comapred to the mass, not the speed of light (the c.) That is just a proprtionality constant. Okay, let’s punch in some real numbers. To keep it simple, we’ll use a mass of one gram. In keeping with the basic metric theme, we’ll calculate c in terms of meters per second: 299,792,458. C2 therefore is 89,875,517,873,681,764. We multiply this by our mass of 1 gram and find that E = 89,875,517,873,681,764 units. Now is the unit in question a gram/meter2/second2? In other words the amount of energy acting on a mass of one gram which is being accelerated at a speed of one meter per second in a time of one second over a distance of one meter (ie one thousandth of a joule). So the energy equivalent of a mass of one gram is 89,875,517,873,681.764 joules? ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 18, 2010, 4:53am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/148 "2010-05-18T04:53:04Z")
Assuming you’ve done the calculator punching right, it seems so. You’re right to notice that using grams means you have to move the decimal smaller by three places, as you’ve been using grams instead of kg. Interesting, because I was told “on the internet” that the Hiroshima bomb destroyed about a gram of uranium, but wiki says the yield was 6.3 E13 Joules, which is off “one gram” by over 10x, the official estimate being smaller. Curse you, internet!!! In any case, it means that if you could totally destroy a paper clip over New York, pretty much the whole place would be burned or blown over. There’s a lot of energy in matter. It’s a conundrum, a mystery, and a weird duck. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [May 18, 2010, 4:53am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/149 "2010-05-18T04:53:09Z")
Found an online energy converter. Rounding my figure off to 89,875,518 megajoules, I convert this into 24,965,422 kilowatt hours. So I have enough energy to run a 100 watt lightbulb for 10,402,259 days. Which is the approximate equivalent of running 285 such bulbs for a century. ---
**Author:** ![Koxinga](https://avatars.discourse-cdn.com/v4/letter/k/4af34b/32.png) [@Koxinga](https://boards.straightdope.com/u/Koxinga)\ **Post date:** [May 18, 2010, 5:29am UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/150 "2010-05-18T05:29:06Z")
> [@Little\_Nemo](#): > > Okay, let’s punch in some real numbers. > > To keep it simple, we’ll use a mass of one gram. In keeping with the basic metric theme, we’ll calculate c in terms of meters per second: 299,792,458. C2 therefore is 89,875,517,873,681,764. We multiply this by our mass of 1 gram and find that E = 89,875,517,873,681,764 units. Now is the unit in question a gram/meter2/second2? In other words the amount of energy acting on a mass of one gram which is being accelerated at a speed of one meter per second in a time of one second over a distance of one meter (ie one thousandth of a joule). > > So the energy equivalent of a mass of one gram is 89,875,517,873,681.764 joules? OK, now _that_ I can get my head around. Thanks. ---
**Author:** ![pretend\_my\_name\_is\_witty](https://avatars.discourse-cdn.com/v4/letter/p/b9e5f3/32.png) [@pretend\_my\_name\_is\_witty](https://boards.straightdope.com/u/pretend_my_name_is_witty)\ **Post date:** [May 18, 2010, 12:18pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/151 "2010-05-18T12:18:15Z")
After having spent four years in the OTC, due to transfer to a proper TA unit as a Potential Officer, I’ve given lots of order for various encounters - including detailed map briefs. Get me on the spot, however, and tell me that something is east or west and I have to roll my eyes back a bit, put my hand out in front of me and recite “Never Eat Shredded Wheat” to work out that West is left and East is right. I _almost_ know it instinctively, and know that I live in the West-End of my City so can figure it out, but it’s always fastest and easiest if I do the stupid rhyme. Think it might come and bite me in the ass when I go for commission - sure way to lose peoples’ confidence in my abilities, I think! ---
**Author:** ![adhay](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@adhay](https://boards.straightdope.com/u/adhay)\ **Post date:** [May 18, 2010, 12:35pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/152 "2010-05-18T12:35:58Z")
> [@pretend\_my\_name\_is\_witty](#): > > … Get me on the spot, however, and tell me that something is east or west and I have to roll my eyes back a bit, put my hand out in front of me and recite “Never Eat Shredded Wheat” to work out that West is left and East is right. I _almost_ know it … I feel your pain. Put the question to me in a new context and I refer to a virtual map of the US. I can remember that CA is on the west coast. ---
**Author:** ![Rushgeekgirl](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rushgeekgirl/32/3715_2.png) [@Rushgeekgirl](https://boards.straightdope.com/u/Rushgeekgirl)\ **Post date:** [May 18, 2010, 1:05pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/153 "2010-05-18T13:05:59Z")
I really should know Spanish given how often I’m exposed to it. I do pick up a lot but I can’t speak in spanish at all. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 18, 2010, 2:24pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/154 "2010-05-18T14:24:58Z")
A number of times I’ve headed the wrong way because “east is right and west is left”, except that I’m driving south… ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [May 18, 2010, 3:26pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/155 "2010-05-18T15:26:37Z")
I don’t know how electricity works. I mean, I got an A in physics A level, and I can tell you what will happen if you connect up such-and-such a circuit, and know that Power equals Ivy Watts and all that stuff, but … how does it actually _work_? I mean, the electricity comes into my house from the power station, and it goes out of my house to the power station again, as it’s a complete circuit, and it does useful stuff in my house, but how? There’s not “less” electricity flowing out of the house than there is flowing in, is there? And in any case, it’s AC, so the electricity is actually flowing into my house then out of my house in the same direction and back again, 50 times a second. How the hell does it do anything useful? ---
**Author:** ![zamboniracer](https://avatars.discourse-cdn.com/v4/letter/z/e9a140/32.png) [@zamboniracer](https://boards.straightdope.com/u/zamboniracer)\ **Post date:** [May 18, 2010, 3:38pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/156 "2010-05-18T15:38:47Z")
I may have to turn in my man card, because I’m no good at grilling food, fixing cars, playing golf or poker. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [May 18, 2010, 8:49pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/157 "2010-05-18T20:49:37Z")
> [@](#): > > OK, now that I can get my head around. Thanks. Thank me only if I got it right. Bouv was saying that’s not the way the equation works. Which I don’t understand because I though all equations worked the same. ---
**Author:** ![Autolycus](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Autolycus](https://boards.straightdope.com/u/Autolycus)\ **Post date:** [May 18, 2010, 9:04pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/158 "2010-05-18T21:04:35Z")
Oh, here’s something. I wasn’t aware that condoms apparently have a proper inside and outside. Honestly, besides lubrication and spermicide, I’m not quite sure what it matters, nor do I know how to tell right from wrong. ---
**Author:** ![Illuminatiprimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/illuminatiprimus/32/11520_2.png) [@Illuminatiprimus](https://boards.straightdope.com/u/Illuminatiprimus)\ **Post date:** [May 18, 2010, 9:22pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/159 "2010-05-18T21:22:11Z")
> [@Autolycus](#): > > Oh, here’s something. I wasn’t aware that condoms apparently have a proper inside and outside. Honestly, besides lubrication and spermicide, I’m not quite sure what it matters, nor do I know how to tell right from wrong. Speaking as a fellow man, don’t you have problems rolling the condom on if you’re doing it the wrong way? That’s how you know it’s wrong! ---
**Author:** ![Autolycus](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Autolycus](https://boards.straightdope.com/u/Autolycus)\ **Post date:** [May 18, 2010, 9:24pm UTC](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836/160 "2010-05-18T21:24:19Z")
> [@Illuminatiprimus](#): > > Speaking as a fellow man, don’t you have problems rolling the condom on if you’re doing it the wrong way? That’s how you know it’s wrong! I haven’t used one in a while (tear), but I don’t recall having any problems either way. Maybe I was just doing it the right way most of the time. [Previous page](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836.md?page=7) [Next page](https://boards.straightdope.com/t/painful-confession-time-things-you-ought-to-know-but-dont/537836.md?page=9)