Lesson 6.4 · Time and Measurement
Mass
Which is heavier, a backpack full of books or an empty backpack? You can feel the difference when you lift them. Mass tells you how much matter is in an object, and you can measure it with two units: grams and kilograms.
Grams and kilograms
Definition
Grams and kilograms
A gram (g) is a small unit of mass. A paper clip has a mass of about 1 gram.
A kilogram (kg) is a much bigger unit of mass. A large book has a mass of about 1 kilogram.
Use grams for light things and kilograms for heavy things.
| Object | About how much mass |
|---|---|
| A paper clip | 1 g |
| A grape | 5 g |
| An apple | 150 g |
| A large book | 1 kg |
| A cat | 4 kg |
| A third grader | 30 kg |
Measuring with a balance
A pan balance compares the mass of two things. The side with more mass goes down. When both sides are level, the masses are equal.
To find the mass of an object, put it on one side. Add gram or kilogram weights to the other side until the balance is level. Then add up the weights.
Common mistake
Big does not always mean heavy! A beach ball is bigger than a rock, but the rock has more mass. Don't judge mass by size. Think about what the object is made of, or measure it.
Solving mass problems
Solve mass problems just like liquid volume problems. Add or subtract to put masses together, take some away, or compare. Multiply for equal masses, and divide to split a mass equally. Write g or kg in your answer.
Worked example: Choose the unit
Would you measure the mass of a strawberry in grams or kilograms?
A strawberry is very light, about as light as a few paper clips. Use grams.
Worked example: Use a balance
A toy car balances with 3 weights. Each weight is 50 g. What is the mass of the toy car?
There are 3 equal weights of 50 g, so multiply: . The toy car has a mass of 150 g.
Worked example: Compare masses
A dog has a mass of 32 kg. A cat has a mass of 5 kg. How much more mass does the dog have?
To compare, subtract: . The dog has 27 kg more mass than the cat.
Tip
Read your answer again and ask: "Does this make sense?" If you find that a pencil is 5 kg, something went wrong. A pencil should be just a few grams.
Practice
Which unit would you use to measure the mass of a watermelon?
Which is the best estimate for the mass of a pencil?
A bag of apples has a mass of 6 kg. A bag of oranges has a mass of 9 kg. What is the total mass of both bags, in kilograms?
Type a number.
A toy robot balances with 4 blocks. Each block has a mass of 20 g. What is the mass of the robot, in grams?
Type a number.
Nora has 45 g of seeds. She puts the same mass of seeds into each of 5 bags. How many grams of seeds go in each bag?
Type a number.
An airline lets each suitcase have a mass of up to 23 kg. Dad's suitcase has a mass of 17 kg. How many more kilograms can he pack?
Type a number.
A beach ball and a brick are about the same size. Which one most likely has more mass?
An empty box has a mass of 3 kg. Maria puts 6 bags of flour in the box. Each bag has a mass of 2 kg. What is the total mass of the box and the flour, in kilograms?
Type a number.