Why Same-Size Units?

Bigger units give a smaller number.

10 problems · about 11 minutes · 1.MD.A.2

What this lesson teaches

The child measures one object with two different units and explains why the numbers differ.

From earlier lessons. Loosen up before the new work.

6 problems

Review — Measuring With Cubes: A stick takes 6 cubes laid end to end. How long is it?
Answer 6 cubes
Why 6 cubes long.
Review — Putting Three in Order: Ropes are 3, 9 and 6 cubes. How long is the longest?
Answer 9
Why 9 is the longest.
Review — Using a Third Thing: A rope is shorter than a string. The string is shorter than a pole. Is the rope shorter than the pole?
Answer Yes.
Why Rope is under string, string is under pole, so rope is under pole.
Review — Longer Than, Shorter Than: Why must you line up one end before comparing?
Answer Otherwise you cannot tell which is longer.
Why Lining up the ends is what makes the comparison fair.
Review — True or False to Twenty: Is 13 − 9 = 2 + 2 true?
Answer True. Both are 4.
Why Both sides come to 4.
Review — Missing Numbers to Twenty: 8 + ? = 12
Answer 4
Why 8 + 4 = 12.

Straightforward practice. Get the method working first.

3 problems

Why must you say which unit you used?
Answer Because a number alone does not say how long.
Why 8 cubes and 8 blocks are different lengths, so the unit matters.
A rope is 6 big blocks. In small cubes, would the number be more than 6?
Answer Yes, more than 6.
Why Smaller units always give a bigger count.
If you swap to smaller units, does the number go up or down?
Answer Up. You need more of them.
Why Smaller units cover less each.

Mixed problems. Work out what kind of question it is before you start.

1 problem

Two Rulers: A desk is 10 small cubes long. Measured in big blocks, will the number be bigger or smaller?
Answer Smaller, because each block covers more.
Why Fewer big blocks are needed to cover the same desk.