seegongsik
Grade 4 (age 9-10)

grade 4 Weight Measurement

A pan balance, like the old scales in a market, hangs a pan from each end of a beam, and the beam sits level when the two pans weigh the same. So if you put an object on one pan and weights you know on the other, and adjust the weights until the beam is level, the weights added together give the object's weight. Weight is the force with which Earth pulls on an object, so on the Moon, where gravity is weaker, it drops to about one sixth; but mass, the amount of matter in the object, is the same everywhere. In the first picture, change the object and the weights and watch the heavier side go down; in the second, compare weight and mass on Earth and on the Moon.

Weighing with a pan balance

50g
30g
⚖️ How to use a pan balance
①First check that the beam is level with both pans empty
②Put the object on the left pan and weights on the right pan
③If the weights' side rises, add weights; if it sinks, take some off
④When the beam is level, the weights added together equal the object's weight

Weight and mass

🌍 Weight and mass are different
①Mass is the amount of matter making up an object, so it is the same everywhere
②Weight is the force with which Earth or the Moon pulls on an object, so it changes from place to place
③The Moon's gravity is about one sixth of Earth's, so on the Moon weight is about one sixth too
④A spring scale measures the pulling force, so its reading drops on the Moon
⑤A pan balance compares the two pans, so on the Moon it balances with the same weights as on Earth

Summary

Weighing with a pan balance
When level: weight of the object = total of the weights
Compare the object with known weights to find its weight
Units of weight
g (grams), kg (kilograms): 1 kg = 1000 g
The units of weight we use most in daily life
Weight and mass
Weight (pulling force) ≠ mass (amount of matter)
Weight changes from place to place, but mass is the same everywhere
🎯 Key points
①A pan balance finds an object's weight by comparing it with weights
②When the beam is level, both sides weigh the same
③We use g and kg as units of weight
④Weight is the force with which Earth pulls on an object, so it is smaller on the Moon
⑤Mass is the amount of matter, so it never changes from place to place

Examples and Unit Test

Example 1
You put an object on one pan of a pan balance and 30 g and 20 g weights on the other, and the beam is level. How much does the object weigh?
1
When the beam of a pan balance is level, both sides weigh the same.
2
The object's weight equals the total of the weights: 30 g + 20 g = 50 g.
▸ 50 g
Set the object to 50 g and the weights to 50 g in the first picture and the beam becomes level.
Example 2
A person with a mass of 60 kg goes to the Moon. What happens to their weight and their mass? (The Moon's gravity is about one sixth of Earth's.)
1
Mass is the amount of matter in the body, so it does not change from place to place.
2
Weight is a pulling force, so on the Moon, where gravity is about one sixth, the weight also drops to about one sixth of Earth's.
▸ Mass stays 60 kg / weight drops to about one sixth of Earth's
On the Moon your body feels light and you can jump high, but the amount of matter in it stays the same.
Unit-test style
Which statement about weight and mass is correct?
①Mass changes from place to place
②Weight is the force with which Earth pulls on an object
③On the Moon an object's mass drops to one sixth
④When a pan balance is level, the object is heavier than the weights
⑤Weight and mass mean exactly the same thing
▸ ② Weight is the force with which Earth pulls on an object
1
Weight is the force with which Earth pulls on an object, so where gravity is different, weight is different too.
2
Mass is the same everywhere (①·③), a level balance means both sides weigh the same (④), and weight is a force while mass is an amount of matter, so they are different (⑤).
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