Hello.
Am having real trouble teaching conversion of measurements in length, weight and volume. Explaining whether the numbers get bigger or smaller when converting, as well as which way the decimal point moves etc.
Any tips out there?
Thanks
Bindiya
Agree: *take your time*
There’s a bit of an intuitive conflict sometimes in the “you want to get to a smaller unit so you make a bigger number” idea. But that idea is central to every ratio in the book.
It’s also extremely helpful to have some sense of what inches and centimeters and kilograms *are* so that when you say you’ve got a pill that’s 350 kilograms… you know something is wrong with that picture unless it’s a strange horror flick :)
First, conceptual: picking up the decimal point and moving it is a very counterproductive approach. Yes, AFTER you have a general idea of how both numbers and measurements work it’s a “quick trick” that can get you results fast. But as a teaching tool, as you have re-discovered for the millionth time, it’s misleading, meaningless, and frustrating.
One point for those with directional confusion: if you move the decimal point rightm the numbers move left, and vice versa. You can get so tangled up in this you’ll go crazy. So avoid it!!
OK, how to make this meaningful.
(1) Measure lots of things in both small units (centimeters or inches) and large units (meters or feet/yards)
(2) Talk about the fact that the different measures mean the *same* thing, and that units of measure are vital.
(3) Look at concrete things, like the dining table for example; it’s 2 meters OR 200 cm long (6 feet OR 72 inches). Notice that when you use smaller pieces, it takes more small pieces (cm or in) to go the length of the table than it takes large pieces. More small pieces = fewer large pieces. (note on vocabulary - *fewer* for countables. Don’t say less. No point in teaching incorrect English .)
So when you go from feet to inches, will you need more or fewer? Answer, more because they’re smaller. OK, multiply by 12. When you go from meters to centimeters, will you need more or fewer? Answer, lots more, because they’re a lot smaller. OK, multiply by 100. Note I’m talking in terms of *multiplication* here (and for the opposite direction, division) Multiplication and division are *mathematical* operations. Picking up a decimal point and moving it is a form of magic and mysticism, which is why it cannot be made to make sense.
Yes, this all takes longer than the “quick trick”. **The first time**. But this approach makes sense, teaches/accesses reasoning and thinking skills, and attaches to the real world. It will need to be taught once and quickly reviewed a couple of times. The magical mystical quick trick will need to be taught in Grade 3 and re-taught in Grade 4 and in Grade 5 … and on forever, as it will not be retained.