Lesson 14: Measuring Weight - 1

For Grade 5 Students (Ages 10-11) - Let's learn about how heavy things are!

What we will learn today:

In this lesson, you will learn to:

  • Identify common units of weight (grams and kilograms).
  • Convert between kilograms and grams.
  • Add and subtract weights.
  • Solve simple word problems involving weight.

Introduction to Measuring Weight

Hello, young mathematicians! Have you ever lifted a bag of rice or a small fruit? Some things are heavy, and some are light. To describe how heavy something is, we measure its **weight** (or mass).

Weight tells us how much matter is in an object. We use special units and tools like scales to measure weight.

Let's learn about these units and how to use them!

Units of Weight

The most common units of weight we use are:

  • **Gram (g):** Used for measuring small weights, like a feather, a paperclip, or a small amount of spice.
  • **Kilogram (kg):** Used for measuring heavier weights, like a bag of sugar, a person's weight, or a sack of potatoes.

Key Conversion:

  • $1$ kilogram (kg) = $1000$ grams (g)

Converting Units of Weight

We often need to convert between kilograms and grams.

Example 1: Convert $3$ kilograms to grams.

Since $1$ kg = $1000$ g, then $3$ kg = $3 \times 1000 = 3000$ g.

Example 2: Convert $4500$ grams to kilograms and grams.

Since $1000$ g = $1$ kg, then $4500$ g = $4000$ g + $500$ g = $4$ kg $500$ g.

Exercise 1: Convert the following units.

$5$ kg = $\underline{\hspace{2cm}}$ g

$7000$ g = $\underline{\hspace{2cm}}$ kg

$2$ kg $300$ g = $\underline{\hspace{2cm}}$ g

$6250$ g = $\underline{\hspace{1cm}}$ kg $\underline{\hspace{1cm}}$ g

Adding and Subtracting Weights

When adding or subtracting weights, make sure they are in the same unit. If not, convert them first! Remember to regroup (carry over or borrow) $1000$ g as $1$ kg, or $1$ kg as $1000$ g.

Example 3: Add $1$ kg $400$ g and $2$ kg $300$ g.

Add kilograms: $1$ kg + $2$ kg = $3$ kg
Add grams: $400$ g + $300$ g = $700$ g
Total: $3$ kg $700$ g

Example 4: Subtract $1$ kg $800$ g from $4$ kg $200$ g.

We cannot subtract $800$ g from $200$ g. So, we borrow $1$ kg from $4$ kg (making it $3$ kg) and add $1000$ g to $200$ g, making it $1200$ g.
Now, $1200 - 800 = 400$ g.
And $3 - 1 = 2$ kg.
Total: $2$ kg $400$ g

Exercise 2: Add or subtract the following weights.

$4$ kg $250$ g + $3$ kg $150$ g = $\underline{\hspace{1cm}}$ kg $\underline{\hspace{1cm}}$ g

$5$ kg $700$ g + $2$ kg $600$ g = $\underline{\hspace{1cm}}$ kg $\underline{\hspace{1cm}}$ g

$8$ kg $900$ g - $3$ kg $400$ g = $\underline{\hspace{1cm}}$ kg $\underline{\hspace{1cm}}$ g

$7$ kg $100$ g - $2$ kg $500$ g = $\underline{\hspace{1cm}}$ kg $\underline{\hspace{1cm}}$ g

Problem Solving with Weight

Let's solve some real-life problems involving weight.

Exercise 3: Solve the following word problems.

  1. A bag of potatoes weighs $5$ kg. A bag of onions weighs $2$ kg $500$ g. What is the total weight of both bags?

  2. A baby weighed $3$ kg $200$ g at birth. After one month, the baby gained $800$ g. What is the baby's weight now?

  3. You bought $1$ kg of apples. You used $350$ g to make a pie. How many grams of apples are left?

Answers to Practice Time

Let's check your answers!

  1. Exercise 1: Converting Units

    • $5$ kg = $5000$ g
    • $7000$ g = $7$ kg
    • $2$ kg $300$ g = $2300$ g
    • $6250$ g = $6$ kg $250$ g

  2. Exercise 2: Adding and Subtracting Weights

    • $4$ kg $250$ g + $3$ kg $150$ g = $7$ kg $400$ g
    • $5$ kg $700$ g + $2$ kg $600$ g = $8$ kg $300$ g (since $700+600=1300$ g = $1$ kg $300$ g)
    • $8$ kg $900$ g - $3$ kg $400$ g = $5$ kg $500$ g
    • $7$ kg $100$ g - $2$ kg $500$ g = $4$ kg $600$ g (since $100 < 500$, borrow $1$ kg from $7$ kg, making it $6$ kg and $1100$ g. $1100-500=600$ g, $6-2=4$ kg)

  3. Exercise 3: Problem Solving with Weight

    • $5$ kg + $2$ kg $500$ g = $7$ kg $500$ g. Total weight is $7$ kg $500$ g.
    • $3$ kg $200$ g + $800$ g = $4$ kg. The baby's weight is $4$ kg. (since $200+800=1000$ g = $1$ kg)
    • $1$ kg = $1000$ g. $1000 - 350 = 650$ g. So, $650$ g of apples are left.

You've completed Lesson 14!

Fantastic work, Math Lanka students! You've learned about measuring weight and how to work with different units. Keep practicing, and you'll be able to weigh anything!

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