8
Practical lesson
Textbook: pp. 20–21
GoalPractise measuring lengths and using scale in real situations.
New words
hand span · qarichpace · qadammap scale · xarita masshtabiinch · duym
Explanation
In this practical lesson we reinforce ways of measuring length. If you measure your hand span and pace once and remember them, you can estimate lengths without a ruler. To measure a long distance with a short ruler or a matchbox, lay it end to end and add the results. On a map, measure the distance between two cities with a ruler and multiply by the scale. Screen diagonals are given in inches: 1 inch = 2.54 cm, so 10 inches is 25.4 cm. Do all practical measuring in the classroom or a safe part of the school yard, away from roads.
Worked examples
A pace is 60 cm. 50 paces are 60 · 50 = 3000 cm = 30 m.
Map scale 1 cm = 40 km. 9.5 cm ↔ 9.5 · 40 = 380 km.
Class activity
“Measuring the room”: groups measure the classroom length in different ways (spans, paces, a 30 cm ruler), write the results in metres and compare. Why do they differ a little?
Practice
1
Your span is 18 cm. A desk is 5 spans long. How many cm?
90
2
On a map the distance is 6 cm, scale 1 cm = 30 km. Real distance in km?
180
3
A corridor was measured with a 30 cm ruler laid 14 times with nothing left over. How long is it in metres?
4.2 m
4
Why is it useful to repeat a long measurement several times?
Each measurement has a small error; repeating reduces its effect.