Measuring length and time

前置知识

2 项必需的前置知识（一直追溯到底）

你将学会

• Compare and order objects by length, directly and by using a third object.
• Measure lengths by laying copies of a unit end to end without gaps or overlaps, and then with a ruler.
• Tell and write time to the hour and half hour on analogue and digital clocks.

定义

• A unit is a fixed amount used for measuring, such as a centimetre or the length of a paper clip.
• The length of an object is the number of units that fit along it end to end.

定理

• Measuring the same length with a smaller unit gives a larger number.
• If A is longer than B and B is longer than C, then A is longer than C.

常见误解

• Starting to measure from 1 on a ruler instead of from 0.
• Leaving gaps or overlaps between units.
• Thinking a string becomes shorter when it is bent.

通向

可选发现

• Fractions and decimals Lengths that fall between whole units lead to fractions of a unit.
• Estimation and rounding Every physical measurement is an approximation, accurate only to some precision.