Rocket drops a pebble into the measuring cup with a splash. "I want to know how big this pebble is," he says.
"Big how?" Raven asks. "Heavy, or wide?"
"Both!" Rocket says. He fishes the pebble out and sets it on the kitchen scale. "Forty grams. That is its mass."
Raven looks at the measuring cup. The water line sits higher than before the splash. "What do you notice about the water?"
"It went up," Rocket says. "The pebble pushed it up."
Nova hovers beside the cup, her light steady. "The water moved exactly as much as something else," she says. "What took up that space?"
Rocket grins. "The pebble. So the water tells us the pebble's volume."
Raven writes two columns in the Bench Log: Mass and Volume.
Mass and volume measure different aspects of matter. Mass is the amount of matter. Volume is the amount of space it takes up.
Mass is measured with a balance, in kilograms or grams. Volume is measured in cubic meters, cubic centimeters, liters or milliliters.
How you measure volume depends on the state of the matter.
| What you measure | What it tells you | Tool | Units |
|---|---|---|---|
| Mass | How much matter is in the object | Balance or kitchen scale | kilograms, grams |
| Volume | How much space the object takes up | Measuring cup, ruler, water displacement | liters, milliliters, cubic centimeters |
| Weight | The force of gravity pulling on the object | Depends on mass and on gravity | pounds |
A room 5.0 meters long, 3.0 meters wide and 2.5 meters high has a volume of 37.5 cubic meters. That is the volume of air inside it.
For an odd shaped solid, put water in a measuring container and read the level. Then lower the object in and read the level again.
The difference between the two readings is the volume of the object.
In one textbook example, the water alone measured 4.8 milliliters. With a toy dinosaur in it, the water measured 5.6 milliliters. So the dinosaur's volume was 0.8 milliliters.
A balance compares the matter in two objects. If the pans sit level, the two sides have the same mass.
Weight depends on the strength of gravity as well as on mass. Earth's gravity is about the same everywhere on Earth.
So at the Bench, more mass always means more weight. A 10 kilogram object weighs ten times as much as a 1 kilogram object, 22 pounds.
| Statement | True or false? |
|---|---|
| A measuring cup measures the volume of a liquid. | ? |
| The volume of a gas is the volume of its container. | ? |
| Multiply length, width and height to find the volume of a box. | ? |
| The displacement method measures mass. | ? |
| Weight depends on the strength of gravity. | ? |
Careful measuring. Tomorrow is your first Bench Lab: you will weigh air and measure a pebble with water.