Comparing Weight

1Summary 重點摘要

Objects have weight. Feel them by hand, or use a scale.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Feel with your handsPut one thing in each hand and lift them slowly at the same time; the heavier side sinks down, and this is the most direct way to feel weight. Example: hold a watermelon in one hand and an orange in the other; the hand with the watermelon sinks, so the watermelon is heavier. Why does it sink? Because the earth pulls heavier things more strongly, and the hand feels a bigger force. This way is easy, but it fails when things are too heavy to lift or their weights are very close, and then we must turn to tools.
2. Step 2: Use a balanceA balance has a left pan and a right pan; put one thing on each pan and the pan that sinks holds the heavier thing, which is a comparison made by the principle of balance. Example: the watermelon sits on the left pan and the orange on the right; the left pan sinks slowly, so the watermelon is heavier, and if the two pans stay level, the two things weigh the same. The balance needs no numbers, so the heavier side can be seen directly. A common mistake is judging before the balance settles; wait until the pointer or the pans are still before reading the result.
3. Step 3: Use a scalePut the thing on a scale and the screen shows the weight as a number at once, which is fast and exact. Example: the watermelon shows 5 kg and the orange shows 0.2 kg; kg is the short form of kilogram, the unit for weighing. Press the zero button first so the screen reads 0, and then put the thing on. A common mistake is ignoring the unit, or putting the thing on before the scale is zeroed, so the reading comes out bigger or smaller than the true weight, and the number is no longer exact.
4. Step 4: Compare the numbersAfter both weights are read as numbers, comparing weight becomes comparing numbers: with the same unit, the bigger number is heavier. Example: the watermelon is 5 kg and the orange is 0.2 kg; 5 is greater than 0.2, so the watermelon is heavier. When weights with decimals are compared, look at the whole-number part first; since 5 is greater than 0, no more looking is needed, and only when the whole parts are equal do we compare the decimal places one by one. A common mistake is thinking 0.2 is big because two digits are written; in fact 0.2 is less than 1 and far smaller than 5.
5. Step 5: Big is not heavySomething that looks big is not always heavy, because some things are hollow inside or made of loose, light material. Example: a big balloon may weigh only 0.05 kg, lighter than an orange of about 0.2 kg; on the other hand, a tiny iron nail can be heavier than a ball of cotton. So never judge weight by size alone; lift the things with your hands or put them on the scale. A common mistake in exercises is choosing 'heavier' whenever the picture is bigger; remember, size and weight are two different matters.
6. Step 6: Know the unitsTwo units are often used for weight: the gram and the kilogram, written g and kg, and 1 kg equals 1000 g. Example: a sweet weighs about 5 g, a textbook about 300 g, and a bag of rice 5 kg. Use grams for light things so the numbers are not too small, and kilograms for heavy things so the numbers are not too large. A common mistake is ignoring units when comparing, for example comparing 500 g with 5 kg by the digits and thinking 500 is bigger; in fact 500 g is only 0.5 kg, far lighter than 5 kg.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Heavy things press your hand down.
• On a balance, the sinking side is heavier.
• A scale shows the number in kg.
• Big is not always heavy; weigh to know.
• Compare weights by comparing kg numbers.