Quit Moving the Decimal Point!
Posted on 17. Mar, 2011 by Peter Price in Decimals, Podcast, Teaching Math, Worksheets
Place value slides are a fantastic resource to teach what happens when numbers are multiplied or divided by a power of 10 (10, 100, 1000 or 0.1, 0.01, 0.001, etc.). The “traditional” method of teaching students to “move the decimal point” is crap, and should NEVER be taught. But to listen to lots of young adults (such as many of my preservice teacher students), that is exactly what they have been told. And unless they understand what I teach in this video, that’s what their students will learn.
Decimal points do not move. Ever. Even in a bad math lesson. The decimal point exists to separate ones from tenths, or whole number places from decimal fraction places. Telling students to move the decimal point is like asking them to rearrange the letters of the alphabet because it happens to suit the teacher’s strange ideas of alphabetical order.
Use a place value slide to demonstrate the “shifting” of every digit left or right as a number is multiplied or divided by a power of ten. Making a place value slide involves use of a sharp knife, and so should be done by an adult. But the effort is worth it in demonstrating clearly what happens within the base ten system when multiplying by a power of ten, such as when converting metric measurements or calculating with percentages.
Download:
- Place Value Slide & Worksheets – PDF file containing Place Value Slide template and 8 worksheets with Answer Keys, ready to use with your class to teach multiplication and division by powers of ten.
Please leave a comment below – the feedback will help us plan future podcast episodes!
Judith
10. Apr, 2012
To move the decimal point was the way I was taught.
A more multisensory way of teaching is to allocate students a digit with one student being the decimal point and then having the students do the “sliding” and the decimal point stays still.
What i find is that students do not understand that our place value system is based on the power of 10. No matter how much work is done with MAB blocks etc they just don’t seem to understand that. Until that is firmly understood they will always have problems with multiplication and division by powers of 10.
Peter Price
10. Apr, 2012
I agree with you 100%, Judith!
Most children (indeed, most adults) don’t appear to really understand the powers of 10 basis for our numeration system. As a result, the question of moving digits or moving the decimal point is rather unimportant.
The idea of having students hold up numerals and a decimal point, and model the multiplication or division process is a great one.
Thanks for your contribution!