[Math] 3rd Grade: Let’s Master Weight (g, kg, t)!
Hello everyone! Today, let’s start studying a new world of math: "Weight." We have already learned about "length" and "capacity" (the amount of water), but now we are going to learn how to represent "how heavy something is" using numbers.
This is a very useful skill for everyday life, whether you’re cooking or weighing your suitcase for a trip. It might feel a bit difficult at first, but once you grab the main points, you'll be fine! Let’s go one step at a time.
1. Learning the Units of Weight (g, kg, t)
When we describe weight, we use a few different "units." First, let’s learn the three basic ones.
① g (gram)
This is the smallest unit. Think of it this way: one 1-yen coin is exactly 1g. It’s very light!
(Examples: The weight of an eraser, the weight of a snack, etc.)
② kg (kilogram)
When you have 1,000 grams, it becomes 1kg. 1kg = 1000g.
Think of it this way: a 1-liter milk carton is about 1kg.
(Examples: Your body weight, a bag of rice, etc.)
③ t (ton)
When you have 1,000 kilograms, it becomes 1t. 1t = 1000kg.
This unit is used when measuring very heavy things.
(Examples: A car, the weight of an elephant, etc.)
★ Here’s a tip!
The word "k" (kilo) means "1,000 times." That’s why 1kg is 1,000 times heavier than 1g. If you remember this, converting between units becomes much easier!
[Summary]
・\(1kg = 1000g\)
・\(1t = 1000kg\)
2. Mastering How to Read a Scale
A tool used to measure weight is called a "scale." There’s a trick to reading the markings on the dial. Let’s try it step-by-step.
Step 1: Check the unit
Look at the face of the scale to see if it measures in "g" or "kg."
Step 2: Find out what "one smallest marking" represents
This is the most important part! For example, if there are 10 markings between 0 and 100g, one marking is 10g.
The way to calculate this is: (Difference between numbers) ÷ (Number of markings).
Step 3: Read where the needle is pointing
Starting from the nearest number, count the markings one by one.
[Common Mistakes]
It is a mistake to assume that "one marking is always 1g!" Depending on the scale, one marking could be 2g, 10g, or even 50g. Get into the habit of asking yourself, "How many grams is one marking on this scale?" at the very beginning!
[Trivia: An Important Preparation Before Measuring]
Before using a scale, always check that the needle points exactly to "0" when nothing is on it. This is called "zero-point adjustment."
3. Calculating Weight (Addition and Subtraction)
You can add and subtract weights just like regular numbers. However, there is one rule you must follow.
Rule: Units must match before calculating!
Calculate "g" with "g," and "kg" with "kg."
Example 1: \(400g + 500g = 900g\)
Example 2: \(1kg 200g + 500g = 1kg 700g\)
[Watch out for carrying over!]
If it goes over 1,000g, convert it into 1kg.
Example: \(800g + 400g = 1200g\) → 1kg 200g
[Summary]
When calculating, be mindful of the "unit wall." Once you have 1,000g, carry over 1 to the kg "room" next door.
4. Developing a Sense for Weight (Estimation)
Before you actually measure something, guessing "I think it’s about this heavy" is called "estimation." Once you can do this, math becomes even more fun!
・Weight Benchmarks for Familiar Objects
1. 1g ・・・ One 1-yen coin
2. 100g ・・・ About one mandarin orange
3. 1kg ・・・ One milk carton, or about 3 to 4 Japanese language textbooks
4. 10kg ・・・ About two school backpacks (randoseru) filled with school supplies
Let’s try it!
Is a pencil case about 10g? Or about 300g?
The answer is... most are around 200g–400g. It’s easier to tell if you imagine how many 1-yen coins it would weigh!
[A Final Message]
Great job on finishing the lesson on weight!
When rewriting units (like \(1kg 50g = 1050g\)), people often forget the zero, so be careful. If you practice enough, you’ll be able to read scales and do the math in a snap. Trying to guess the weight of things in your home and then measuring them is excellent practice. I’m cheering for you!