Lesson 77: Mass and Weight

New Concept

The mass of an object is how much matter an object has. Weight is the measure of the force of gravity on that object.

Why it matters

Understanding the difference between mass and weight teaches you that quantities can depend on their context, a key idea in advanced science. Mastering unit conversions builds the foundational skill of proportional reasoning, which is essential for solving complex algebraic equations.

What’s next

Next, you’ll practice converting between U.S. Customary units of weight and metric units of mass.

The mass of an object is how much matter an object has. Weight is the measure of the force of gravity on that object. Though an object's weight depends on the force of gravity, its mass does not. For example, an object's weight on the moon is less, but its mass remains the same.

An astronaut has a mass of 80 kg on Earth. On the moon, her mass is still 80 kg, but her weight is much less. A bowling ball's mass is constant, but it would weigh less on Mars than on Earth due to different gravitational forces.

Think of it this way: your mass is how much 'stuff' you are made of, which never changes. Your weight is how hard gravity pulls on that stuff. On the moon, the pull is weaker, so you weigh less, but you are still the same you! Weight changes with location, mass is constant.

The units of weight in the U.S. Customary System are ounces, pounds, and tons. Ounce is abbreviated oz. Pound is abbreviated lb.

A baby elephant weighs 200 pounds. How many ounces is that? $200 \times 16 \text{ oz} = 3200 \text{ oz}$. A small car weighs 1.5 tons. How many pounds is that? $1.5 \times 2000 \text{ lb} = 3000 \text{ lb}$.

Think of these as small, medium, and large weight measurements. An ounce is very light, like a few slices of bread. It takes 16 of those to equal one pound, the weight of a loaf of bread. For really heavy things, like a car, we use tons, which is 2000 pounds!

Grams and kilograms are metric units of mass. The prefix kilo- means 'thousand.' A kilogram is 1000 grams. Gram is abbreviated g. Kilogram is abbreviated kg.

A dictionary has a mass of 2 kilograms. How many grams is that? $2 \times 1000 \text{ g} = 2000 \text{ g}$. A pineapple has a mass of 1500 grams. How many kilograms is that? $1500 \div 1000 = 1.5 \text{ kg}$. Choose the more reasonable measure for a cat: 4 g or 4 kg. A cat would be about 4 kg.

The metric system keeps it simple with thousands. A gram is super light; a single dollar bill has a mass of about one gram. A kilogram, which is 1000 grams, is much heavier, like a big textbook or a bag of sugar. Just remember 'kilo' means one thousand, and you're all set to convert!