Timeline for Why is negative times negative = positive?
Current License: CC BY-SA 3.0
10 events
| when toggle format | what | by | license | comment | |
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| Nov 26, 2015 at 2:48 | history | post merged (destination) | |||
| Nov 10, 2015 at 4:09 | history | edited | zahbaz | CC BY-SA 3.0 | added 8 characters in body |
| Nov 9, 2015 at 21:38 | comment | added | zahbaz | Let us continue this discussion in chat. | |
| Nov 9, 2015 at 21:16 | comment | added | David K | There is a huge difference between "grossly simplified" and "just plain wrong". It's the difference between "ignore air resistance" and "assume kinetic energy is $mv$". The buoy indeed is a much better example, still possibly a bit complicated but at least it actually works the way you need in order to make a correct analogy with multiplication of negatives. | |
| Nov 9, 2015 at 21:05 | comment | added | zahbaz | Or if you will, replace the word ice cube by weight, glass by buoy, and temperature by height above sea level. It's just an architecture to express opposites. | |
| Nov 9, 2015 at 20:42 | comment | added | zahbaz | @DavidK No, we should suppose that the scenario is grossly simplified. Children can understand that sort of disclaimer. The point isn't to teach physics, but to paint a mental image of what opposites mean in terms of multiplication. We can use a metaphorical example as a heuristic, no? I get your point, but I find this example works well enough for pedagogical purposes.. If you want a physical example, then take the ratio of the mass of your ice cube and water to be approx 1:100. Then, yes, one ice cube will (eventually) lower the temperature of the drink by about 1 degree C. | |
| Nov 9, 2015 at 19:23 | comment | added | David K | What should we pretend? Should we pretend that when you remove some of the ice, the liquid in the glass warms up? That's a "false intuition" of the sort I meant. There are so many analogies that actually work in real life, why not just use one of them instead of making up plausible (to a naive person) but incorrect physics? | |
| Nov 9, 2015 at 15:55 | comment | added | zahbaz | @DavidK Just use the word "pretend." I doubt the child is ready for a talk about specific heat. | |
| Nov 9, 2015 at 13:49 | comment | added | David K | Can you really warm up a drink by removing ice cubes from it? In reality, an ice cube in a drink absorbs some heat from the drink, and some of the ice may melt and become part of the liquid in the drink, then mix with the other liquid, lowering its temperature. In either case, when you remove the ice, you do not undo either of those effects; the drink remains cooler than before you put the ice in it. So you either have to make some complicated "averaging" argument or you have to rely on a false physical intuition to be "unlearned" later. | |
| Nov 9, 2015 at 8:08 | history | answered | zahbaz | CC BY-SA 3.0 |